Feature/parallel co2 tests
What this MR does / why does DuMux need it: Adds communication for boxVolume output and adds four new co2 tests with and unstructured grid with saturation changes over process boundaries. Additionally increase the injection rate and harmonizes the output by lowering the initial timestepsize and introducing a max TimeStepSize for the tests.
Which issue this MR fixes: fixes #819 (closed)
Special notes for your reviewer: Check on as many machines as possible if the output actually is the same. If i remember correctly the co2-tests have been prone to fail with different compiler versions before.