Bugfix inc hyd

This fixes issues #972 The solution is to allow vertical diffusion even in cells that are distorted. If not, effects of bottom friction are correctly taken into account and velocities are completely wrong. I think that for prisms, it is not a problem to allow vertical diffusion, as this happens in a vertical profile only, so it should not be affected by hydrostatically inconsistent elements.

A few notes: I only tested for prismatic elements, not for tetraëders (so i do not know whether also changes are needed to mt02_tt.f). I only tested with hydrostatic flow.

I found that the V test case is affected. Now some spurious currents (with velocities 1E-7 m/s) are generated. That's why I changed the coefficients in the vnv.py file. the currents are weaker though than without the use of the hydrostatic inconsistencies filter.