review sites

[Jakob-Unfried]

• Move from cyten.networks.site to some appropriate place. we probably dont want a cyten.networks submodule
• The current implementation, in particular the charged tensor part is probably not what we want.
• Should establish some framework for few-body operators, that knows how to compose them.

Random related ideas:

• MPS should know more about operators. either access to lattice / model or to a special class that is essentially a list of sites but has operators and N_sites_per_ring?
• Devise classes for on-site operators and for few-body-operators.
  There is a class of "elementary" operators (classification depends on what is conserved!) that have virtual legs with only a single sector and are thus the natural building blocks for MPOs. but we could also support linear combinations thereof.