pep8 and typo fix for tutorials

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

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

doc/source/images/4-graphene.py.diff

doc/source/images/5-superconductor_band_structure.py.diff

doc/source/tutorial/1-quantum_wire_revisited.py

doc/source/tutorial/2-quantum_well.py

doc/source/tutorial/2-spin_orbit.py

doc/source/tutorial/4-graphene.py

doc/source/tutorial/5-superconductor_band_structure.py