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/astnodes.py

@@ -310,6 +310,7 @@ class Block(Node):
 
     def insert_before(self, new_node, insert_before):
         new_node.parent = self
+        assert self._nodes.count(insert_before) == 1
         idx = self._nodes.index(insert_before)
 
         # move all assignment (definitions to the top)
@@ -337,6 +338,7 @@ class Block(Node):
         return tmp
 
     def replace(self, child, replacements):
+        assert self._nodes.count(child) == 1
         idx = self._nodes.index(child)
         del self._nodes[idx]
         if type(replacements) is list: