Autofeaturizer needs ability to use custom column names

[ardunn]

Autofeaturizer should have the ability to use custom column names. "structure" and "composition" should not be hardcoded in as they are now.

The autofeaturizer init method should accept either two arguments, one for identifying the composition column and one for identifying the structure column:

__init__(...composition_col="composition", structure_col="structure"...)

or a single dict argument named colmap (or similar) which maps multiple columns onto their featurization arguments:

colmap={"structure": "MyStructure", "composition": "MyComposition"}

This should also be done for bandstructure and dos, although they are less important