Feature/site ordering

In FiniteSystems the sites are now stored lexicographically

In InfiniteSystems the sites are stored in the order defined by sorted(sites_with_neighbors) + sorted(internal_sites) + sorted(interface) where internal_sites are the sites that only have hoppings inside the fundamental domain, sites_with_neighbors is the complement of internal_sites within the fundamental domain, and interface is the interface sites (outside the fundamental domain).

For the simple case where all the sites in the fundamental domain have hoppings outside the fundamental domain this will correspond to lexicographical ordering over the sites in the fundamental domain.

This merge request supercedes !5 (closed).