vectorize systems so that value functions may be called once
on a whole vector of sites (for onsite value functions) or a pair of
vectors of sites (for hopping value functions). In this way we avoid
the Python overhead of calling the value functions many times. For systems
with vectorization enabled this can result in a ~100x speedup when evaluating
the Hamiltonian.

See merge request kwant/kwant!323
Closes kwant/kwant#302