only accept value functions with named positional arguments

doc/source/code/figure/closed_system.py.diff

doc/source/code/figure/quantum_well.py.diff

kwant/_common.py

kwant/builder.py

kwant/operator.pyx

kwant/tests/test_builder.py

kwant/tests/test_operator.py

kwant/tests/test_wraparound.py

kwant/wraparound.py