use a coverage configuration's possible values as a way to describe it

[ricardogsilva]

Lets make it so that a coverage configuration's possible_values can be used to describe the coverage configuration and make it searchable.

What we would like is for the frontend to be able to:

• discover which configuration parameters exist (model, time window, scenario, etc.)
• discover which coverage configurations exist, based on inputting values for possible values

As an example, as request like:

api/v2/coverages/coverage_identifiers?scenario=rcp26&model=model_ensemble

would return all coverage identifiers that have scenario=rcp26 and model=model_ensemble as possible values