maybe write more specific operations for MyChem chembl.drug_mechanisms data

[colleenXu]

Can use BioThings filter/jmespath parameters to write more specific operations for this data, using chembl.drug_mechanisms.action_type values (last compiled here). It'd be similar to what was done before with drugcentral.bioactivty.action_type

Could be useful for MVP2 chem-affects-gene.

Old notes in biothings/pending.api#100 (comment)