Mpi fixes

This should fix issue #28.

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?

See merge request !17


Former-commit-id: 79fbeba71a8f2e5e6e3986f09302bc304880d947