Error when levelsfmt not defined in Configuration File for NonEquilibrium calcul
<!--- Provide a general summary of the issue in the Title above -->
## Context
~/.radis
```
[DB_ISSUE_80]
info = ... 
path =
        PATH/TO/cdsd_hitemp_07
format = cdsd-hitemp
parfuncfmt = hapi
```
Note that levelsfmt is not declared

Minimal code:
```
s = calc_spectrum(wavenum_min=2050, 
            wavenum_max=2100,
            Tgas=4500, 
            Trot=4499,
            Tvib=5001,
            pressure=1.01325,       # bar
            mole_fraction=1e-3,
            isotope='1,2,3,4',
            path_length=1,        # cm
            databank='DB_ISSUE_80',
            )
```
## Expected Behavior
levelsfmt is not declared --> we could expect that it use the RADIS built-in spectroscopic constants by default.

## Current Behavior
Crashes later with:
```
Traceback (most recent call last):
  File "<ipython-input-33-79986cf35618>", line 11, in <module>
    databank='DB_ISSUE_80',
  File "/home/corentin/radis/radis/lbl/calc.py", line 411, in calc_spectrum
    name=name,
  File "/home/corentin/radis/radis/lbl/factory.py", line 960, in non_eq_spectrum
    self._check_noneq_parameters(vib_distribution, singleTvibmode)
  File "/home/corentin/radis/radis/lbl/base.py", line 1535, in _check_noneq_parameters
    calc_Evib_harmonic_anharmonic=calc_Evib_harmonic_anharmonic
  File "/home/corentin/radis/radis/lbl/base.py", line 1619, in _calc_noneq_parameters
    df, calc_Evib_harmonic_anharmonic=calc_Evib_harmonic_anharmonic
  File "/home/corentin/radis/radis/lbl/base.py", line 394, in _add_EvibErot
    + "format: {0}".format(self.params.levelsfmt)
NotImplementedError: Impossible to calculate Evib Erot with given energy format: None
```
## Possible Solution
If None, use default values (i.e. RADIS built-in spectroscopic constants) as `levelsfmt = radis`







diff --git a/docs/lbl/lbl.rst b/docs/lbl/lbl.rst
index 2eb29fb0..eeaa3312 100644
--- a/docs/lbl/lbl.rst
+++ b/docs/lbl/lbl.rst
@@ -414,7 +414,8 @@ partition functions, for instance::
                                      # file. If `hapi`, then HAPI (HITRAN Python
                                      # interface) is used to retrieve them (valid if
                                      # your databank is HITRAN data). HAPI is embedded
-                                     # into RADIS. Check the version.
+                                     # into RADIS. Check the version. If not specified then 'hapi'
+                                     # is used as default
     parfunc = PATH/TO/cdsd_partition_functions.txt
                                      #  path to tabulated partition function to use.
                                      # If `parfuncfmt` is `hapi` then `parfunc`
diff --git a/radis/lbl/loader.py b/radis/lbl/loader.py
index dc6d2c60..b1e67154 100644
--- a/radis/lbl/loader.py
+++ b/radis/lbl/loader.py
@@ -694,7 +694,7 @@ class DatabankLoader(object):
             format to read tabulated partition function file. If ``hapi``, then
             HAPI (HITRAN Python interface) [1]_ is used to retrieve them (valid if
             your database is HITRAN data). HAPI is embedded into RADIS. Check the
-            version.
+            version. If partfuncfmt is None then ``hapi`` is used. Default ``hapi``.
         parfunc: filename or None
             path to tabulated partition function to use.
             If `parfuncfmt` is `hapi` then `parfunc` should be the link to the
@@ -706,7 +706,7 @@ class DatabankLoader(object):
             how to read the previous file. Known formats: (see :data:`~radis.lbl.loader.KNOWN_LVLFORMAT`).
             If ``radis``, energies are calculated using the diatomic constants in radis.db database
             if available for given molecule. Look up references there.
-            If None, non equilibrium calculations are not possible. Default ``None``.
+            If ``None``, non equilibrium calculations are not possible. Default ``'radis'``.
         db_use_cached: boolean, or ``None``
             if ``True``, a pandas-readable csv file is generated on first access,
             and later used. This saves on the datatype cast and conversion and
@@ -821,7 +821,7 @@ class DatabankLoader(object):
             format to read tabulated partition function file. If ``hapi``, then
             HAPI (HITRAN Python interface) [2]_ is used to retrieve them (valid if
             your database is HITRAN data). HAPI is embedded into RADIS. Check the
-            version.
+            version. If partfuncfmt is None then ``hapi`` is used. Default ``hapi``.
         parfunc: filename or None
             path to tabulated partition function to use.
             If `parfuncfmt` is `hapi` then `parfunc` should be the link to the
@@ -1073,7 +1073,7 @@ class DatabankLoader(object):
             format to read tabulated partition function file. If ``hapi``, then
             HAPI (HITRAN Python interface) [1]_ is used to retrieve them (valid if
             your database is HITRAN data). HAPI is embedded into RADIS. Check the
-            version.
+            version. If partfuncfmt is None then ``hapi`` is used. Default ``hapi``.
         parfunc: filename or None
             path to tabulated partition function to use.
             If `parfuncfmt` is `hapi` then `parfunc` should be the link to the
@@ -1085,7 +1085,7 @@ class DatabankLoader(object):
             how to read the previous file. Known formats: (see :data:`~radis.lbl.loader.KNOWN_LVLFORMAT`).
             If ``radis``, energies are calculated using the diatomic constants in radis.db database
             if available for given molecule. Look up references there.
-            If None, non equilibrium calculations are not possible. Default ``None``.
+            If ``None``, non equilibrium calculations are not possible. Default ``'radis'``.
         db_use_cached: boolean, or ``None``
             if ``True``, a pandas-readable csv file is generated on first access,
             and later used. This saves on the datatype cast and conversion and
@@ -1134,6 +1134,11 @@ class DatabankLoader(object):
         # %% Check inputs
         # ---------
 
+        # use radis default for calculations non equilibrium calculations
+        # if the levelsfmt is not specified in the databank
+        if levelsfmt is None:
+            levelsfmt = "radis"
+
         (
             name,
             path,
@@ -2044,17 +2049,15 @@ class DatabankLoader(object):
         isotope = int(isotope)
 
         # Use HAPI (HITRAN Python interface, integrated in RADIS)
-        if parfuncfmt == "hapi":
+        # no tabulated partition functions defined. Only non-eq spectra can
+        # be calculated if energies are also given
+        if parfuncfmt == "hapi" or parfuncfmt is None:
             parsum = PartFuncHAPI(
                 M=molecule, I=isotope, path=parfunc, verbose=self.verbose
             )
         elif parfuncfmt == "cdsd":  # Use tabulated CDSD partition functions
             assert molecule == "CO2"
             parsum = PartFuncCO2_CDSDtab(isotope, parfunc)
-        elif parfuncfmt is None:
-            # no tabulated partition functions defined. Only non-eq spectra can
-            # be calculated if energies are also given
-            parsum = None
         else:
             raise ValueError(
                 "Unknown format for partition function: {0}".format(parfuncfmt)
['radis/lbl/loader.py', 'docs/lbl/lbl.rst']