disable OpenMP when doing MPI tests; it would just slow everything down

.gitlab-ci.yml

@@ -19,8 +19,8 @@ test_min:
     - python setup.py install --user -f
     - python3 setup.py install --user -f
     - nosetests
-    - mpiexec --allow-run-as-root -n 4 nosetests -x
-    - mpiexec --allow-run-as-root -n 4 nosetests3 -x
+    - OMP_NUM_THREADS=1 mpiexec --allow-run-as-root -n 4 nosetests -x
+    - OMP_NUM_THREADS=1 mpiexec --allow-run-as-root -n 4 nosetests3 -x
     - nosetests3 -x --with-coverage --cover-package=nifty2go --cover-branches
     - >
       coverage report | grep TOTAL | awk '{ print "TOTAL: "$6; }'