[VnV][telemac3d] Update vent example with better choices for a few keywords

- TREATMENT OF NEGATIVE DEPTHS = 2 (new default value since release 9.0) and
TIDAL FLATS = YES (default value) to improve mass conservation of water
- use of gradient conjugate + preconditioning 34 = 2*17 to solve PPE, more
efficient than GMRES for this example
- accuracy for propagation decreased to 1.E-15 (improved differences between
sequential and parallel runs)
- switch to non-hydrostatic version