add_batch volume of the single point
It looks like the recent change with self.sampler.update(0)
leads to issues: see below

```
 File "/home/skoposov/pyenv38/lib/python3.8/site-packages/dynesty/dynamicsampler.py", line 1595, in run_nested
    passback = self.add_batch(nlive=nlive_batch,
  File "/home/skoposov/pyenv38/lib/python3.8/site-packages/dynesty/dynamicsampler.py", line 1715, in add_batch
    for results in self.sample_batch(nlive_new=nlive,
  File "/home/skoposov/pyenv38/lib/python3.8/site-packages/dynesty/dynamicsampler.py", line 1079, in sample_batch
    bound = self.sampler.update(0)
  File "/home/skoposov/pyenv38/lib/python3.8/site-packages/dynesty/nestedsamplers.py", line 689, in update
    self.mell.update(self.live_u[:, :self.ncdim],
  File "/home/skoposov/pyenv38/lib/python3.8/site-packages/dynesty/bounding.py", line 618, in update
    firstell = bounding_ellipsoid(points, pointvol=pointvol)
  File "/home/skoposov/pyenv38/lib/python3.8/site-packages/dynesty/bounding.py", line 1410, in bounding_ellipsoid
    raise ValueError("Cannot compute a bounding ellipsoid to a "
ValueError: Cannot compute a bounding ellipsoid to a single point if `pointvol` is not specified.
```
The investigation shows that this happens when the  subset https://github.com/joshspeagle/dynesty/blob/b3dbd52369a9b9172b20e76cb53ddd7f43bf9a1c/py/dynesty/dynamicsampler.py#L1035
has only one element and therefore the ellipsoid has only one point. 
It is still not clear to me what value is the most appropriate there. So for my own purposese I've temporarily set the volume to
math.exp(-1. * len(saved_logl) / nblive)

diff --git a/py/dynesty/dynamicsampler.py b/py/dynesty/dynamicsampler.py
index 216adee..ae3eb45 100644
--- a/py/dynesty/dynamicsampler.py
+++ b/py/dynesty/dynamicsampler.py
@@ -1077,10 +1077,11 @@ class DynamicSampler(object):
 
             live_logl_min = min(live_logl)
             if self.sampler._beyond_unit_bound(live_logl_min):
-                # notice we are setting the volume to zero
+                # notice we are setting the volume to some value
                 # as it is really not clear what the volume should
-                # be
-                bound = self.sampler.update(0)
+                # be TODO
+                pointvol = math.exp(-1. * len(saved_logl) / nblive)
+                bound = self.sampler.update(pointvol)
                 if save_bounds:
                     self.sampler.bound.append(copy.deepcopy(bound))
                 self.sampler.nbound += 1
['py/dynesty/dynamicsampler.py']