Skip CF DSG checks if dataset is not DSG
Currently, the CF checker returns failures for DSG related checks that do not apply.  In our current framework, they should be skipped, not executed.

diff --git a/compliance_checker/cf/cf.py b/compliance_checker/cf/cf.py
index 47c7fa2..b5a9f8d 100644
--- a/compliance_checker/cf/cf.py
+++ b/compliance_checker/cf/cf.py
@@ -1,4 +1,5 @@
 import re
+from functools import wraps
 from collections import defaultdict, OrderedDict
 import numpy as np
 
@@ -189,6 +190,17 @@ def map_axes(dim_vars, reverse_map=False):
 
     return dict(ret_val)
 
+# helper to see if we should do DSG tests
+def is_likely_dsg(func):
+    @wraps(func)
+    def _dec(s, ds):
+        if hasattr(ds.dataset, 'featureType'):
+            return func(s, ds)
+
+        # @TODO: skips if we have formalized skips
+        return None
+
+    return _dec
 
 class CFBaseCheck(BaseCheck):
     """
@@ -2571,6 +2583,7 @@ class CFBaseCheck(BaseCheck):
     #
     ###############################################################################
 
+    @is_likely_dsg
     def check_all_features_are_same_type(self, ds):
         """
         9.1 The features contained within a collection must always be of the same type; and all the collections in a CF file
@@ -2688,6 +2701,7 @@ class CFBaseCheck(BaseCheck):
 
         return Result(BaseCheck.HIGH, valid)
 
+    @is_likely_dsg
     def check_orthogonal_multidim_array(self, ds):
         """
         9.3.1 The orthogonal multidimensional array representation, the simplest representation, can be used if each feature
@@ -2715,7 +2729,7 @@ class CFBaseCheck(BaseCheck):
                         ret_val.append(result)
         return ret_val
         
-
+    @is_likely_dsg
     def check_incomplete_multidim_array(self, ds):
         """
         9.3.2 The incomplete multidimensional array representation can used if the features within a collection do not all have
@@ -2745,9 +2759,8 @@ class CFBaseCheck(BaseCheck):
                         return []
 
         return ret_val
-        
-        
 
+    @is_likely_dsg
     def check_contiguous_ragged_array(self, ds):
         """
         9.3.3 The contiguous ragged array representation can be used only if the size of each feature is known at the time
@@ -2779,6 +2792,7 @@ class CFBaseCheck(BaseCheck):
 
         return ret_val
 
+    @is_likely_dsg
     def check_indexed_ragged_array(self, ds):
         """
         9.3.4 The indexed ragged array representation stores the features interleaved along the sample dimension in the data
@@ -2810,6 +2824,7 @@ class CFBaseCheck(BaseCheck):
 
         return ret_val
 
+    @is_likely_dsg
     def check_feature_type(self, ds):
         """
         9.4 A global attribute, featureType, is required for all Discrete Geometry representations except the orthogonal
@@ -2835,6 +2850,7 @@ class CFBaseCheck(BaseCheck):
             ret_val.append(result)
         return ret_val
 
+    @is_likely_dsg
     def check_coordinates_and_metadata(self, ds):
         """
         9.5 Every feature within a Discrete Geometry CF file must be unambiguously associated with an extensible collection
@@ -2934,7 +2950,7 @@ class CFBaseCheck(BaseCheck):
 
         return ret_val
 
-
+    @is_likely_dsg
     def check_missing_data(self, ds):
         """
         9.6 Auxiliary coordinate variables (spatial and time) must contain missing values to indicate a void in data storage
['compliance_checker/cf/cf.py']