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/kernelcreation.py

@@ -345,9 +345,9 @@ def create_staggered_kernel_2(assignments, **kwargs):
         direction = direction_string_to_offset(direction)
         for d, s in enumerate(direction):
             if s == 1:
-                ghost_layers[d][1] = 0
-            elif s == -1:
                 ghost_layers[d][0] = 0
+            elif s == -1:
+                ghost_layers[d][1] = 0
 
     def condition(direction):
         """exclude those staggered points that correspond to fluxes between ghost cells"""