replace possible_hopppings by HopppingKind (only the code is done)

doc/source/reference/kwant.builder.rst

kwant/builder.py

kwant/graph/tests/test_slicer.py

kwant/lattice.py

kwant/linalg/tests/test_mumps.py

kwant/solvers/tests/_test_sparse.py

kwant/tests/test_builder.py

kwant/tests/test_plotter.py