Use string comparisons where supported

Author: dhomeier

specutils/io/parsing_utils.py

@@ -104,11 +104,10 @@ def spectrum_from_column_mapping(table, column_mapping, wcs=None):
 
             if not isinstance(cm_unit, u.Unit):
                 cm_unit = u.Unit(cm_unit)
-            cm_type = str(cm_unit.physical_type)
-            if 'length' in cm_type or 'frequency' in cm_type or 'energy' in cm_type:
+            if cm_unit.physical_type in ('length', 'frequency', 'energy'):
                 # Spectral axis column information
                 kwarg_val = kwarg_val.to(cm_unit, equivalencies=u.spectral())
-            elif 'spectral flux' in cm_type:
+            elif 'spectral flux' in str(cm_unit.physical_type):
                 # Flux/error column information
                 kwarg_val = kwarg_val.to(cm_unit, equivalencies=u.spectral_density(1 * u.AA))
         elif tab_unit: