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?
Yes, quite possible. I had in my mind the idea of setting gravity to zero. In that case, the pressure is constant in the box and one should just get a situation where the velocity decays linearly from the inflow to the opposite boundary. In essence, this is a zero-dimensional situation where one can compute everything as an ODE. But because our assumption is that the rhs of the divergence equation is dominated by the hydrostatic pressure, it is going to be zero if gravity=0. Not sure what to do here.