finally,
so the problem was NOT due to mpi4py : this library is NOT required.
However these two are: libopenmpi-dev and openmpi-bin
sudo apt install libopenmpi-dev openmpi-bin
(again) Thanks to:
hydro-informatics.com/get-started/install-telemac.html
Openmpi-bin solved the // issue mpirun:
phmusiedlak@HQ-FIX-PMK:~$ mpirun -np 4 mpi_hello
Hello from HQ-FIX-PMK, rank 0 (of 4).
Hello from HQ-FIX-PMK, rank 1 (of 4).
Hello from HQ-FIX-PMK, rank 2 (of 4).
Hello from HQ-FIX-PMK, rank 3 (of 4).
but it is not enough as we require a fortran compiler: here comes
libopenmpi-dev
Thanks
Pierre-Henri