Merge branch 'fixup/deprecate-norbs' into 'master'

doc/source/pre/whatsnew/1.5.rst

doc/source/tutorial/faq.rst

doc/source/tutorial/first_steps.rst

doc/source/tutorial/graphene.rst

doc/source/tutorial/magnetic_field.rst

doc/source/tutorial/plotting.rst

doc/source/tutorial/spectrum.rst

doc/source/tutorial/spin_potential_shape.rst

doc/source/tutorial/superconductors.rst

kwant/builder.py

kwant/continuum/tests/test_discretizer.py

kwant/linalg/tests/test_mumps.py

kwant/physics/tests/test_dispersion.py

kwant/physics/tests/test_leads.py

kwant/physics/tests/test_noise.py

kwant/solvers/tests/_test_sparse.py

kwant/tests/test_builder.py

kwant/tests/test_comprehensive.py

kwant/tests/test_lattice.py

kwant/tests/test_operator.py