ENH: Atoms.get_spacegroup() method which determines the spacegroup from the Atoms.positions

Add a get_spacegroup method to Atoms, in a way similar to pyspglib.

Methodology: Test space groups from highest to lowest symmetry, and return the first match which keeps the cell unaffected by applying symmetry operators.

Merge request from a cleaned (recent) repo which has testing pipeline OK. This is a re-submission after a few failures. Sorry for that. ;-)

Emmanuel.