Superpose then do absolute
Get rid of n_features as an input
Only do atom indices
Put all functionality in one class with init args

Make both superpose and atom_indices optional
Add doc warnings about superpose modifying traj
Get rid of extra blank line at end
Fix test (remove n_features from init)
Merge pull request #223 from mpharrigan/superabs

Raw Positions Superpose Featurizer