[VnV][waqtel] Update heat_exchange example with better choices for some keywords

- TREATMENT OF NEGATIVE DEPTHS = 2 (new default value since release 9.0)
to improve mass conservation (water depth + temperature, water depth up to
machine accuracy)
- SCHEME FOR ADVECTION OF VELOCITIES = 5 (PSI) rather than 1 (characteristics)
for better mass conservation + reduced differences between seq/par runs
- use of gradient conjugate + preconditioning 34 = 2*17 to solve PPE, more
efficient than GMRES for this example
- accuracies for diffusion of velocities/tracers/k-epsilon + PPE set to
1.E-15 as easily and efficiently reached with this set-up