New sub-module for site featurization and GA model selection

[schlexer]

Featurize site in periodic and non-periodic atomic structures, read-in and save data sets, structures and defect/defect-free structure pairs, compute cohesive energies, run genetic-algorithm based feature selection.

[mhangaard]

Great work!
A few comments:

• There is already a GA feature selection module in CatLearn, so we may have some redundancy here.
• The doc strings are formatted in Google style, whereas the rest of CatLearn is formatted in NumPy style.
• This is a quite long module, and maybe it would be an advantage to split it up in multiple modules (.py files), dividing by imports.

All in all, since this module is all new additions and already in use by other researchers, I think we should go ahead and merge. It is not urgent to deal with the comments above.