make QM molecule partial sanitized with RDKit

This aligns the RDKit conversion process. A relaxed sanitization
process is required to avoid kekulization/sanitization/valence issues
which would prevent a molecule from being created.

Especially relevant in the context of `draw`, which has an RDKit
backend that calls this function. We don't want it to fail drawing
simple because it doesn't follow the sanitization rules.

Author: jonwzheng

rmgpy/qm/molecule.py

@@ -147,7 +147,7 @@ def rd_build(self):
         """
         Import rmg molecule and create rdkit molecule with the same atom labeling.
         """
-        return self.molecule.to_rdkit_mol(remove_h=False, return_mapping=True)
+        return self.molecule.to_rdkit_mol(remove_h=False, return_mapping=True, sanitize="partial")
 
     def rd_embed(self, rdmol, num_conf_attempts):
         """