When checking isomorphism of Molecules, also check the initial map

This fixes the recently added failing isomorphism check.
If you pass in a mapping, it is checked for validity.

Note that this is not done at the Graph level or VF2, only at 
the Molecule level. I.e. this bug probably still exists for Graph
and maybe Group objects.

Author: rwest

rmgpy/molecule/molecule.py

@@ -1450,6 +1450,11 @@ def is_isomorphic(self, other, initial_map=None, generate_initial_map=False, sav
         # check multiplicity
         if self.multiplicity != other.multiplicity:
             return False
+        
+        # if given an initial map, ensure that it's valid.
+        if initial_map:
+            if not self.is_mapping_valid(other, initial_map, equivalent=True):
+                return False
 
         if generate_initial_map:
             initial_map = dict()