Merge branch 'terhorst-nest-version-bump' into 'master'

Update NEST to 3.1

See merge request technical-coordination/devops/platform/ebrains-spack-builds!33

spack.yaml

@@ -15,7 +15,7 @@ spack:
     - py-seaborn %gcc@10.3.0
     - py-matplotlib %gcc@10.3.0
     # EBRAINS simulators
-    - nest@3.0 +python +gsl +mpi %gcc@10.3.0
+    - nest@3.1 +python +gsl +mpi %gcc@10.3.0
     - arbor +mpi ^python@3:3.9 %gcc@10.3.0
     - neuron +mpi %gcc@10.3.0
     - py-pynn@0.9.6 %gcc@10.3.0