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?
We did not find a better solution either. A way to handle this would be to improve the formulation to treat the compressibility on the left hand side, with all the trouble that results out of this (breaking symmetry of the matrices, nonlinearity etc.). Some mantle convection codes do that, e.g. Terra. However this is definitely beyond the scope of a small pull request like this, and we found this state to probably be the best, we can achieve for now. 
This way it works for cases in which our assumption is valid, it ignores the cases where we are slightly deviating from our assumption and it gives a convergence failure in cases where we are deviating significantly.