PP_geo and (P)PF : parallel PGD (and PREP?)

[AlexandreMary]

As of CY50 (and in specific branches on older cycles), PGD now should be able to run in parallel (MPI). It would be interesting to set Davai PGD tests parallel to run faster.

It requires:

• to compile with macro -DSFX_MPI
• to run with namelist key LSFX_MPI=.T. (unless this is the default in PGD and PREP ? to be checked)
• to switch attribute engine="blind" to engine="parallel" in the AlgoComponent of the test script and set up a number of MPI tasks (?)

Another proposal is to set NHALO_PREP=20 in PREP to speed it up (cf. comment in #26)