adding serialization for AtomGraph and AtomFeat types

[rkurchin]

So that we can store graphs as files more easily, etc.

[rkurchin]

@SeanXiaoyuSun if you could review this for merge that would be great. High-level summary of changes is:

• added tests that serialize/deserialize work for AtomGraph
• added a batch processing function to build (and optionally featurize) graphs from CIFs and serialize them to file (may make sense to make a similar thing for SMILES (or rename the function and do it multiple-dispatch style!), especially if you end up building DeepChem's graphcon in the near future)
• made both of the custom types nonparametric as I don't think it was adding anything and was causing some issues in implementing some other functionality...we could potentially put this back in the future if it seems important
• added some notes to the readme about how I'm using terminology and also tried to clean up some variable names to be consistent with that usage

[SeanXiaoyuSun]

So it ends up with replacing JLD2 with JSON to save metadata right? And you need to resolve the conflicts as well.

[rkurchin]

Nope, JSON is just for reading in that feature_info file. I'm using Serialization for actually saving the files (.jls). You good for me to merge this once I fix the conflicts though?

[SeanXiaoyuSun]

yeah all tests passed on my end