[WIP] Tutorial for running Atomic Convolutions

[nitinprakash96]

This is currently a work in progress. This tutorial will cover:

• the basic structure of the Atomic Conv model.
• featurize a selection of the pdbbind dataset and train a simple ACNN model.
• examples, if needed

[coveralls]

Coverage increased (+0.1%) to 81.276% when pulling 396dc2a on nitinprakash96:acnn into 30b0354 on deepchem:master.

[rbharath]

@nitinprakash96 Would you mind if I merged this tutorial in as-is? I can use this as a base to flesh out some material as I work on improving the tutorials.

[nitinprakash96]

@rbharath Sure. Please go ahead.