add an `norbs` attribute to `SiteFamily`s
This allows us to reason about the size of the Hamiltonian without having to evaluate it. This should be optional to maintain compatibilty, but will become compulsory in Kwant2.
This allows us to reason about the size of the Hamiltonian without having to evaluate it. This should be optional to maintain compatibilty, but will become compulsory in Kwant2.