[VnV][telemac3d] Update tetra example with better choices for a few options

- TREATMENT OF NEGATIVE DEPTHS = 2 (new default value since release 9.0) and
TIDAL FLATS = YES (default value) to improve mass conservation of water
- SOLVER FOR PPE = 1 (conjugate gradient) + PRECONDITIONING FOR PPE = 34
(= 2*17) which combo can be used with finer accuracy with similar CPU time
as default SOLVER FOR PPE = 7 (GMRES)