Mpi fixes

This should fix issue #28 (closed).

Tested it with the SodShock example, was able to reproduce the bug and catch the exact location using the address sanitizer. Have not yet tested it in a "real" parallel environment, e.g. a cosma run, but only on my laptop.

Could you test this with the CosmoVolume and verify that it indeed no longer crashes?

Checked with the test that failed in #28 (closed), that needed the force rebuild hack to fail. These commits work past that so accepting.