Move LuaMara to modules/unigrid.lua
Correct some Mara sources for Linux builds
Remove unused Mara code and have Mara build respect MPI presence
Bix build bug on older Mac's: add ranlib call for .a files
Fix some warning messages that appear in the Mara source on Linux
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.
Add the 2d implosion test from the Athena test page
Add #include <cstring> to Mara/subgrids.cpp
Add c2p failure output to plm-split to be consistent with weno-split
Add GravityArray to Mara class and set_gravity function to Lua interface
Put hacked (but working) gravity support into Mara; MPI might not work