Move MatrixOfEquivalentPositions to Python side

Magnus Rahmrequested to merge
move-matrix-of-equivalent-positions-to-python-side into master

The class MatrixOfEquivalentPositions has some fairly trivial tasks that don't seem particularly crucial performance wise. In this MR, I moved it to the Python side. I don't see any significant change in performance, not even in benchmark/profile_matrix_of_equivalent_positions.py. There is no change in interface whatsoever.

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