enh: speeded up atomic overwriting in geometries and Atoms class
enh: speeded up atomic overwriting in geometries and Atoms class

This enables easy re-assignment of atoms in big structures.

Signed-off-by: Nick Papior <81a73bca3d342ffee33d7cf845fdbf77b6553831@gmail.com>

zerothi/sisl,zerothi/sisl