OpenMPI: Use ROMIO for IO

[ax3l]

A note on using libSplash with OpenMPI:

OpenMPI's default for its IO backend is OMPIO, starting with 2.x.

Unfortunately, that backend contains severe bugs leading to data corruption and sporadic crashes as of the latest releases (e.g. 3.1.3 and 4.0.0). Please see open-mpi/ompi#6285 for details.

As a work-around for all systems that rely on OpenMPI (and its derivatives, such as BullMPI), disable the "OMPIO" default IO backend and fallback to the existing ROMIO backend for MPI-I/O until fixed versions are available.

Available runtime switches:

export OMPI_MCA_io=^ompio
mirun ...

or

mpirun --mca io ^ompio ...

Other MPI implementations such as MPICH, and MPICH-based flavors such as IntelMPI, use ROMIO by default (they develop ROMIO) and are not affected.

[ax3l]

Issue continued in: openPMD/openPMD-api#446