use `syst` for Kwant systems instead of `sys`

doc/source/images/ab_ring.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_transport.py.diff

doc/source/pre/whatsnew/1.0.rst

doc/source/pre/whatsnew/1.1.rst

doc/source/tutorial/ab_ring.py

doc/source/tutorial/closed_system.py

doc/source/tutorial/graphene.py

doc/source/tutorial/plot_graphene.py

doc/source/tutorial/plot_zincblende.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