support for inchikeys (issue #131); test data not updated yet

[vineetbansal]

Changes towards addressing issue #131

• ability to add inchi keys when we're saving smiles (used in preprocessing and creation of test/train sets).
• ability for precalculated inchi keys for pubchem (not added there yet though).
• a short run of the workflow has been checked for completion without errors (on the LOTUS_truncated dataset).
• None of the test_data files have been updated yet, so tests are expected to fail.

[vineetbansal]

closing this in favor of PR #136