duplicated drug mechanism records in CHEMBL

[erikyao]

E.g. on CHEMBL1201631 Compound Report Card, there is only one drug mechanism record. However, http://mychem.info/v1/chem/CHEMBL1201631 shows two.

The duplication comes from the original source file, https://www.ebi.ac.uk/chembl/api/data/mechanism.json.

Same problem may occur to drug indication records.

Suggested fix: remove duplicates after chembl_parser.py#L52; pay attention to nested lists when hashing records.