Merge branch 'solve_split' into devel

doc/source/images/ab_ring.py.diff

doc/source/images/graphene.py.diff

doc/source/images/quantum_well.py.diff

doc/source/images/spin_orbit.py.diff

doc/source/reference/kwant.rst

doc/source/reference/kwant.solvers.rst

doc/source/tutorial/ab_ring.py

doc/source/tutorial/graphene.py

doc/source/tutorial/quantum_well.py

doc/source/tutorial/quantum_wire.py

doc/source/tutorial/quantum_wire_revisited.py

doc/source/tutorial/spin_orbit.py

doc/source/tutorial/superconductor_transport.py

doc/source/tutorial/tutorial1.rst

doc/source/tutorial/tutorial2.rst

doc/source/tutorial/tutorial5.rst

doc/source/whatsnew/1.0.rst

examples/advanced_construction.py

examples/square.py

kwant/__init__.py