Investigate if REF works with ESMValTool Obs data pool

Depends on #295. Pause refinement of this issue until the basics of observation access are understood.

Investigation needed to understand if REF can easily work with the Tier 2 and Tier 3 observation data.
TODO: add link to esmvaltool tier documentation.

If we added an ESMValTool recipe to the REF, and it used data in ESMValTool Tier 2 or 3, i,e, not in Obs4MIPs or Obs4REF, would it work? Would it contravene the REF principles / prevent the recipe being added to the central REF repository?

Task: Investigate the above questions, summarise conclusions and any recommendations in an issue comment.

Time box 0.5 day

Start by reading the dependency issue #295 and the ESMValTool observation-access context, then investigate how Tier 2 and Tier 3 data would be used by REF recipes. Compare this with the REF principles and central repository requirements. Done means posting a comment summarising whether this works, any principle conflicts, and recommendations.

Written by the indexing model from the issue text.