Bugfix inc hyd

This fixes issues #972 (closed) 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.