What this MR does / why does DuMux need it: Fixes the failing tests (1pni and 3pni for now) by using a fixed increment timestep and lower DtInitial for the tpfa-test. I went for changing the DtInitial in the CMakeLists.txt instead of putting it in the input file. If we think it is better to change the params.input, and by this achieve the same amount of vtk-outputs for tpfa and box, we can also do that.
Which issue this MR fixes: fixes #871 (closed)
Special notes for your reviewer: I have the 1pni version not ready yet as the other tests are also affected by the fixed timestep increment. I will get back to it and push the rest after my Bernd-Meeting... I have not made any progress on the 2p-test so far.