introduce immutable SiteGroup, reduce number of SiteGroup classes

doc/source/images/2-ab_ring.py.diff

doc/source/reference/kwant.lattice.rst

doc/source/reference/kwant.rst

doc/source/tutorial/1-quantum_wire.py

doc/source/tutorial/2-ab_ring.py

doc/source/tutorial/2-quantum_well.py

doc/source/tutorial/2-spin_orbit.py

doc/source/tutorial/3-band_structure.py

doc/source/tutorial/3-closed_system.py

doc/source/tutorial/4-graphene.py

doc/source/tutorial/5-superconductor_band_structure.py

doc/source/tutorial/5-superconductor_transport.py

doc/source/tutorial/tutorial4.rst

kwant/__init__.py

kwant/builder.py

kwant/graph/tests/test_slicer.py

kwant/lattice.py

kwant/linalg/tests/test_mumps.py

kwant/physics/tests/test_dispersion.py

kwant/physics/tests/test_noise.py