Enable specifying NEURON branch for GitLab CI. (#525)

Enable support for CI_BRANCHES specifications in PR descriptions.
The implementation lives in hpc/gitlab-pipelines!17.
Closes #505.

Author: olupton

.gitlab-ci.yml

@@ -14,6 +14,10 @@ stages:
 # Set up Spack
 spack_setup:
   extends: .spack_setup_ccache
+  variables:
+    # Enable fetching GitHub PR descriptions and parsing them to find out what
+    # branches to build of other projects.
+    PARSE_GITHUB_PR_DESCRIPTIONS: "true"
   script:
     - !reference [.spack_setup_ccache, script]
     # This allows us to use the CoreNEURON repository in regression tests of