bug: d*k methods for SOC/NC returned None
bug: d*k methods for SOC/NC returned None

They should return NotImplemented to be more specific.

Fixes #183.

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

zerothi/sisl,zerothi/sisl