Compatibility normal flux

This is code intended to address issue #71 (make_pressure_rhs_compatible). It is currently not working (except for cases where int_Gamma u.n = 0) and it breaks the testcases, because I added a lot of debugging output. I would appreciate some help though, because at the moment, the correction we discussed in issue #71 seems to prevent the models with non-matching boundary conditions from converging. I added a test case (pressure-compatibility-2.prm) that illustrates the problem in case the in- and outflow velocity boundary conditions do not match. Although the integral over the right-hand-side of the pressure block of the stokes equation is equal to the boundary integral over the normal velocity (see debugging output) the solver reports a convergence failure. Funny thing is, if I do the usual correction, assuming int_Gamma u.n = 0 although it is not, the solver does converge and gives a reasonable result, except for the fact that the pressure is not rising over time (as one would expect from the permanent inflow). Maybe I am doing something wrong in the correction of the RHS?
Exactly, this is weird. I first thought of the pressure normalization, but that is switched off.
I guess as long as the inflow is very small (below the solver tolerance) the solver is converging, but does not solve accurately enough to resolve the pressure increase with time, and if the inflow gets large enough to be visible, the solver does not longer converge. This happens for example when setting the inflow to 1e-8 instead of 1e-9 in this test.
Timo and I sat together for an evening during Cider and looked for the reason for the convergence failure, but did not find a solution, except for maybe that our assumption for the compressible stokes equation that the pressure is dominated by the static pressure is not longer valid in case of significant inflow.