Implement more dynamic collections.

[sturoscy-personal]

Collections should not be json files, but rather generated dynamically from the project/collection ID (i.e. CMIP6). Look into to continuing to use esgvoc for this.

[sashakames]

We will need a mechanism to add exceptions to the list of ESGVOC projects, for now namely "EMD" as that does not map to a Collection.

[sturoscy-personal]

See issue #138. Fixed in that issue.