Refactor featurizer

[nissy-dev]

What I did

• Remove old test
• Remove MolecularWeight feturizer
  • I think this featurizer is not necessary to maintain. And, the name of the file includes MolecularWeight and RDKitDescriptors is basic.py, it is not so good... So, I removed MolecularWeight and, renamed the file.
• Refactor base Featurizer
  • Base Featurizer basically override only featurize method
• Remove deepchem.utils.save.log

[rbharath]

This is some nice cleanup @nd-02110114! Swapping over to logging instead of the old log in the remaining spots and renaming featurize_complexes to featurize both are very nice.

I'm not sure whether we should delete MolecularWeightFeaturizer or deprecate it first. It's not a publicly exposed class so it might be fine to delete it.

[rbharath]

I think this is safe to delete since it wasn't exposed in the public API deepchem/feat/__init__.py. MolecularWeight isn't an useful featurizer, and I don't think anyone was actually using it.

@peastman Do you think we're OK to delete this or would it be better to deprecate it instead?

[peastman]

If we really think it's not useful, I'd be fine with just removing it.

[nissy-dev]

CI failure is related to pymatgen type checking

[rbharath]

The only travis failures look to be mypy related so going to ahead and merge this in!