[STATS] Include MPI read and write for the tstat PyTree in FastS.

[antjost]

The ability to use Cmpi.convertFile2PyTree/Cmpi.convertPyTree2File for the tstat PyTree is needed given that this approach is used for all other PyTrees in Cassiopee/Fast environement. Alternatively, the need for a single file per proc for tstat should be clearly visible in the FastS manual (https://onera.github.io/Fast/doc/FastS.html).