Make a 'Lattice' class that is compatible (in some restricted sense) with a symmetry

We will define a class of sites (Lattice) characterized by:

• sym -- the symmetry on which this class of sites is based

• div -- an integer factor to divide sym.periods into a finer lattice

• T -- matrix where rows are the lattice vectors in the basis of sym.periods / div

We will need to update the to_unit_cell, unit_cell and act functions to work with sites from these lattices.

Such lattices should also define an ordering (<) that will determine which sites are chosen for the fundamental domain. This will allow us to remove the bas parameter to the fundamental_domain functions, and use simple site comparison.