Saving sparse matrices

[RichRick1]

@gabrielasd we can use https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.load_npz.html this function for load npz sparse matrix and this https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.save_npz.html function for saving npz sparse matrix

It might be good to add wrap of these function to the the utils.py file for saving/loading not the entire hamiltonian, but one body/ two body terms of it