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?