Ingest MetaRelate generated GRIB/CF mappings.

This PR **must** be merged after #853.

This PR is the final piece in the jigsaw to incorporate the new GRIB/CF phenomenon translations as generated by MetaRelate.

Also see companion Iris Code Generators PR https://github.com/SciTools/iris-code-generators/pull/22.

TODO:
 - [x] This PR **must** be rebased once #853 is merged.