[input validation] nk.graph.Chain([L]) does not fial

Created by: PhilipVinc

nk.graph.Chain([L]) should fail, but does not, and leads to failure later that it took me a while to realise...

We should validate the input to be an integer.

>>> graph = nk.graph.Chain([L])
>>> sgb = graph.space_group_builder()
>>> sgb.little_group([0])