Introduce `InfiniteSquareNetwork` supertype to reduce redundancies?

[leburgel]

Introducing an InfiniteSquareNetwork supertype could cut down on some of the current redundancies and at the same time leave some space for users to add their own objects that can be contracted using CTMRG. I feel a Union would restrict that in unnecessary ways. Also more conceptually, a InfiniteSquareNetwork type would make sense to me since CTMRG should in principle work on any infinite translationally invariant square network. @pbrehmer @lkdvos

Originally posted by @pbrehmer in #111 (comment)

[lkdvos]

This is probably better discussed in #116