detect symmetry equivalent site and avoid duplicating the featurization

[kjappelbaum]

probably better to implement on the featurizer level ...

[kjappelbaum]

in principle we should be able to do it base on find_equivalent_sites of the symmetrized structure in pymatgen. if we write it in serial it is trivial, we just look up in a dictionary if we already have the result for one of the equivalent sites, otherwise we compute it and add it to the dictionary.

[kjappelbaum]

initial version implemented in https://github.com/kjappelbaum/oximachine_featurizer/tree/v0.2.12