Automate the process of finding the real space lattice vectors and site positions of BlochModels

Now kwant.qsymm.model_to_builder takes lattice vectors and site positions as input, but this information is typically already encoded in the Bloch Hamiltonian written in the real space convention, so we could make these arguments optional and automate this step. Also useful for finding the Brillouin-zone of BlochModels, which in turn would allow automating finding candidates for spatial symmetries.