Automate pulling of stash2lfric variable mapping

[jfrost-mo]

What problem does your feature request solve?

The mapping of variables from STASH codes to LFRic in _stash_to_lfric.py comes from a centrally maintained file in MOSRS. We want to ensure it remains up to date, and does not drift.

Describe the solution you'd like

It would be really nice to copy it automatically, so it remains up-to-date.

Describe alternatives you've considered

We also need to consider restricted files, as while the variable mappings are openly licenced we will have other things, like METplus masks, that are going to be sensitive, and thus shouldn't be in the public repository. Perhaps they should use a similar copy method but end up in the restricted repository? We'd have to check if they are suitable for sharing with the partnership, otherwise we might have to directly fetch from MOSRS.

Another consideration is that all MOSRS repositories should be migrated to GitHub over the next year and a half (ish), so thinks might get practically easier then.