Raw Positions Superpose Featurizer

This class featurizes on a subset of atoms, and runs superpose first.

I also got rid of the `n_features` attribute from `RawPositionsFeaturizer`, because it is only there to provide a warning.
Ah yes, I see your point. I'm hesitant to throw a `copy` in there, because I can imagine most of the time it will be unnecessary.

@rmcgibbo do you have any thoughts?