Test restart functionality

With !1039 (merged), the possibility of reading an initial solution from a Vtk file has been introduced. This is implemented quite general, but a model-specific part remains that defines the primary variable names of the model that are expected to be found in the Vtk file. Since this is sensitive to changes of these names in the output routines, it should be tested thoroughly:

• Adapt relevant main files such that the restart functionality will work again.
• Add one restart test in the corresponding CMakeLists.txt.
• Add more parallel tests in test/porousmediumflow/richards/implicit (depends on #535 (closed)).

Examples can be found in test/porousmediumflow/2p/implicit/incompressible, test/porousmediumflow/2p2c/implicit and test/freeflow/navierstokes.