Improve symmetry interface
Right now verifying that the discrete symmetry is broken requires some low-level code:
sym = syst.discrete_symmetry(params=params)
h = syst.hamiltonian_submatrix(sparse=True, params=params)
# replace with "h = syst.cell_hamiltonian(...); t = syst.inter_cell_hopping(...)"
# if the system is infinite
sym.validate(h)
Because this is one of the important operations, we should probably simplify it.