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?
no need to modify, more of a language advise since I see you use this word in other places as well :-) The word "shall" is pretty formal. It is referenced in places like the American constitution in contexts such as
  "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof"
It is also used in places where you intend to be purposefully high-brow, as in Jill: "Wolfgang, you bought the wrong kind of dried cherries, it should have been brand XY." Wolfgang: "I shall make certain that that never happens again!"

I think in a context like here, one would just say "This is a compressible test case intended to test the behavior..."

(Really, not a big deal, and not worth addressing. You know that your English is excellent if people start pointing out these kinds of things :-)