gitlab-ci: Adds (command-line) tool to simplify .gitlab-ci.yml

- Adds `.gitlab/pipeline-ci-tool.sh` to enact most of the stages of the gitlab CI pipeline
  - enables interactive/command-line reproduction of the pipeline
  - `.gitlab/pipeline-ci-tool.sh` is documented in .gitlab/README.md with instructions
    on how to use at the command line and what each function is doing
- All commands formerly in .gitlab-ci.yml are now one-line invocations of `.gitlab/pipeline-ci-tool.sh`
  so .gitlab-ci.yml is now considerably smaller and easier to read with statements like
  `.gitlab/pipeline-ci-tool.sh mrs-compile debug_gnu` or `.gitlab/pipeline-ci-tool.sh check-params gnu`
- Previously, all results were compared again the stored regression answers. This meant that
  any error (e.g. layout) would show up as a fail of all types. We use the regression answers to
  check the repro-symmetric mode and then compare everything else to repro-symmetric or other results
  as appropriate. This allows us to distinguish between types of errors. The GH actions are doing it
  this way, and we originally did this in the first forms of the pipeline, but in the last re-factor
  I lazily switched to using the regression answers for everything.