VTK output: skip non-interior cells

this makes the NCP model work in parallel again...
syncronize the primary values of the overlap after determining the initial solution and after each update

this should make all simulations work properly in parallel...
change the test for the parallel ECFV discretization to use the NCP model and the energy module

these two were not tested in parallel, the immiscible model was...
validate the results produced by the parallel tests

this implied a few changes to the "runtest.sh" script and requires
reference solutions...