Using RD-Filters on MoleculeNet datasets

A number of the MoleculeNet datasets have PAINS compounds and other compounds that detract from their usefulness as benchmarking datasets. Let's use this issue to brainstorm the set of datasets that we want to filter. I think we can use @PatWalters https://github.com/PatWalters/rd_filters library to help us filter datasets.

Off the top of my head, I think we can start by applying the PAINS filters to the

1. Chembl
2. Chemb25
3. HIV
4. PCBA

datasets. We should discuss here to see if this makes sense though.

CC @mufeili @PatWalters @lilleswing @peastman