Reimplement create_staggered_kernel

create_staggered_kernel now takes a list of assignments to staggered fields and otherwise behaves like create_kernel. I have included a diffusion equation test case in both D2Q5 and D2Q9. Putting vectorial/tensorial quantities on the staggered points also works.

This merge requests break API compatibility with the previous create_staggered_kernel.

Fixes #16 (closed). My next step is to implement automatic FV discretization.

Build of dependent projects (pygrandchem, pystencils_walberla) passes as per https://i10git.cs.fau.de/pycodegen/pycodegen/merge_requests/6.