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.

pystencils/cpu/kernelcreation.py

@@ -34,6 +34,7 @@ def create_kernel(assignments: AssignmentOrAstNodeList, function_name: str = "ke
                       transformation :func:`pystencils.transformation.split_inner_loop`
         iteration_slice: if not None, iteration is done only over this slice of the field
         ghost_layers: a sequence of pairs for each coordinate with lower and upper nr of ghost layers
+                      that should be excluded from the iteration.
                      if None, the number of ghost layers is determined automatically and assumed to be equal for a
                      all dimensions
         skip_independence_check: don't check that loop iterations are independent. This is needed e.g. for