[VnV][telemac3d] Update Viollet 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
- use of gradient conjugate + preconditioning 34 = 2*17 to solve PPE, more
efficient than GMRES for this example
- use of preconditioning 34 for diffusion of every variable (more efficient)
- accuracy for PPE and propagation decreased from 10^-10 to 10^-12