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.
