Use --oversubscribe with OpenMPI

Author: ziotom78

.github/workflows/tests.yml

@@ -95,6 +95,6 @@ jobs:
         run: |
             for proc in 1 2 3; do
               echo "Running MPI test ($MPI) with $proc processes"
-              PYTHONPATH=. mpiexec -n $proc python3 ./test/test_mpi.py
-              PYTHONPATH=. mpiexec -n $proc python3 ./test/test_detector_blocks.py
+              PYTHONPATH=. mpiexec --oversubscribe -n $proc python3 ./test/test_mpi.py
+              PYTHONPATH=. mpiexec --oversubscribe -n $proc python3 ./test/test_detector_blocks.py
             done