Fix a major (two-headed) bug having to do with Mara and MPI

1. Mara/Makefile did not build mara.cpp with the DEFINES macro. This
   lead to the subgrids.hpp typedef'ing DecomposedCartesianDomain to
   SimpleCartesianDomain.

2. The result of unigrid:local_mesh_size('shape') cannot be used as
   input to Mara's set_domain function: Mara expects the global domain
   shape as input.
Try a KH setup which is easier to write down, images are still outdated
Add visual.write_ppm function
Add an h52png.lua tool
Update the KH image in TestProblems.org: but WENO5 versions are outdated