Set MPI executable for ParFlow

[mvhulten]

This is needed when GCC+OpenMPI is used. This is tested on JURECA.

With Intel+ParaStationMPI this already went right; there is no effect when compiling with Intel LLVM.

Resolves: #87

---

A potential problem I see with my commit is with systems that don't use Slurm.

[kvrigor]

A potential problem I see with my commit is with systems that don't use Slurm.

Yes. The solution should be generic—at least across all machines defined in default.2025.env:

TSMP2/env/default.2025.env

Lines 35 to 46 in 0252d09

	if [[ $SYSTEMNAME == *"jedi"* || $SYSTEMNAME == *"jupiter"* ]]; then
	export TSMP2_ENV_FILE=${env_dir}/jsc.2025.gnu.openmpi
	elif [[ $SYSTEMNAME == *"jureca"* || $SYSTEMNAME == *"juwels"* || $SYSTEMNAME == *"jusuf"* ]]; then
	export TSMP2_ENV_FILE=${env_dir}/jsc.2025.intel.psmpi
	elif [[ $SYSTEMNAME == *"marvin"* ]]; then
	export TSMP2_ENV_FILE=${env_dir}/uni-bonn.gnu.openmpi
	elif [[ $SYSTEMNAME == *"UBUNTU"* ]]; then
	export TSMP2_ENV_FILE=${env_dir}/ubuntu.gnu.openmpi
	else
	echo "WARNING: Unknown default environment for machine '$(hostname)'"
	known_machine="false"
	fi

[mvhulten]

Addressed better in #90.