Add error handling for CS to CS comparison at different resolutions

CS to CS regridding is not implemented in gcpy. Ref and dev at different
CS resolutions can only be compared on a lat-lon grid. This is an open
issue, but now there is error handling to exit out with a helpful message
if attempting to regrid CS to CS due to the comparison grid.

Signed-off-by: Lizzie Lundgren <elundgren@seas.harvard.edu>

gcpy/benchmark.py

@@ -244,7 +244,7 @@ def compare_single_level(refdata, refstr, devdata, devstr, varlist=None,
         cmpgridtype = 'll'
     else:
         cmpgridtype = 'cs'
-        
+
     # Determine what, if any, need regridding.
     regridref = refres != cmpres
     regriddev = devres != cmpres
@@ -280,12 +280,20 @@ def compare_single_level(refdata, refstr, devdata, devstr, varlist=None,
         if refgridtype == 'll':
             refregridder = make_regridder_L2L(refres, cmpres, weightsdir=weightsdir, reuse_weights=True)
         else:
-            refregridder_list = make_regridder_C2L(refres, cmpres, weightsdir=weightsdir, reuse_weights=True)
+            if cmpgridtype == 'cs':
+                print('ERROR: CS to CS regridding is not yet implemented in gcpy. Ref and dev cubed sphere grids must be the same resolution, or pass cmpres to compare_single_level as a lat-lon grid resolution.')
+                return
+            else:
+                refregridder_list = make_regridder_C2L(refres, cmpres, weightsdir=weightsdir, reuse_weights=True)
     if regriddev:
         if devgridtype == 'll':
             devregridder = make_regridder_L2L(devres, cmpres, weightsdir=weightsdir, reuse_weights=True)
         else:
-            devregridder_list = make_regridder_C2L(devres, cmpres, weightsdir=weightsdir, reuse_weights=True)
+            if cmpgridtype == 'cs':
+                print('ERROR: CS to CS regridding is not yet implemented in gcpy. Ref and dev cubed sphere grids must be the same resolution, or pass cmpres to compare_single_level as a lat-lon grid resolution.')
+                return
+            else:
+                devregridder_list = make_regridder_C2L(devres, cmpres, weightsdir=weightsdir, reuse_weights=True)
 
     ####################################################################
     # Get lat/lon extents, if applicable