fix diffs of tutorial examples

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/plot_graphene.py.diff

doc/source/images/plot_zincblende.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