make axes labeled use A [B] instead of A [in units of B]

TODO

doc/source/images/ab_ring.py.diff

doc/source/images/band_structure.py.diff

doc/source/images/closed_system.py.diff

doc/source/images/graphene.py.diff

doc/source/images/quantum_well.py.diff

doc/source/images/quantum_wire.py.diff

doc/source/images/spin_orbit.py.diff

doc/source/images/superconductor_band_structure.py.diff

doc/source/images/superconductor_transport.py.diff

doc/source/tutorial/ab_ring.py

doc/source/tutorial/band_structure.py

doc/source/tutorial/closed_system.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_band_structure.py

doc/source/tutorial/superconductor_transport.py