Investigate customisability of REF esmvaltool recipe availability

If CMEW needs a recipe that is not desirable for the "standard" config of the REF, is there a mechanism to enable this? REF has plugin functionality - does that relate to this question?

Does the REF constrain the way in which ESMValTool compares models? Does it limit the number of model datasets in recipes generated by `ref solve`? Are plots showing multiple runs together any harder to produce than using ESMValTool directly?

Expect that we can answer this via documentation and/or discussions with REF developers. Out of scope: this investigation does not need to demonstrate the answer - don't need to try the relevant interface functionality.

Time box: 1 day

Start by reviewing the REF plugin functionality and the ESMValTool recipe-generation behavior described in the issue. Consult the existing CMEW documentation and discuss the questions with REF developers, without implementing or demonstrating the interfaces. Done means documenting whether custom recipes, model comparisons, dataset counts, and multi-run plots are constrained.

Written by the indexing model from the issue text.