- In add_site_family, the first argument cannot be a general site family, it must be a lattice from lattice.py. We should rename site_family -> lattice in this context. In fact, that argument could be even a Polyatomic lattice (that is not a site family at all).
- The automatically selected other_vectors could be longer but more orthogonal.
** modes
When on a band edge, the calculation of modes should fail in a clear way, for example by raising kwant.UndefinedResultError (itself derived from ValueError). We could also provide a function (tentative name: kwant.tolerate) that turns such exceptions into None values (useful for plotting).
