Throw error if increment radical on a surface site.

This shouldn't happen, but seems to be somehow.
Hopefully this Assert statement will catch the culprit.

rmgpy/molecule/molecule.py

@@ -411,6 +411,7 @@ def increment_radical(self):
         """
         # Set the new radical electron count
         self.radical_electrons += 1
+        assert not self.is_surface_site(), "Attempted to increment radical count of a surface site."
         if self.radical_electrons <= 0:
             raise gr.ActionError('Unable to update Atom due to GAIN_RADICAL action: '
                                  'Invalid radical electron set "{0}".'.format(self.radical_electrons))