Open source workflow

[jfrost-mo]

Fixes https://github.com/MetOffice/CSET-workflow/issues/146. That issue has quite a lot of design discussion around this change, so is worth a read before reviewing.

Most additions are just what was in the workflow, so doesn't need to much attention. The only change I've made to the code is adding a few more docstrings and fixing a couple minor lints.

The bits that require scrutiny are the restricted files install script, and making sure I've drawn the repository boundaries correctly.

Also note that the install script clones the main branch by default, so you will want to set it to 146_remove_open_sourced_code instead while the excess hasn't been removed.

[github-actions]

[JorgeBornemann]

Everything looks good. The changes to the open vs restricted repositories are as discussed, and to me are very reasonable, but once the other partners start testing and implementing their site-specific information we may have to do some fine adjustment.
One possible future improvement could be to code a wrapper (i.e. cset_run_workflow) for the cylc vip command so the workflow can be constructed and run in one go, but it needs a lot more thinking.
Note that I didn't have time to test the branch in our installation. If something goes wrong we can deal with it in another issue.
I leave to you committing the branches, as you have a good plan already on the order they have to be committed.

[jfrost-mo]

I though about having a wrapper script, but didn't want to hide the cylc 8 commands, as that would make CSET different to most other workflows. I'm open for more discussion and ideas in future though. I'll go ahead and merge this now and we can improve it in follow up issues.