Ingest MetaRelate generated UM/CF mappings.

This PR must be merged **after** #841.

This PR incorporates the new UM/CF translations as generated by MetaRelate, and removes the now redundant ```iris/fileformats/mosig_cf_map.py```.

Also see companion Iris Code Generator PR https://github.com/SciTools/iris-code-generators/pull/21.

Iris test result changes are primarily due to the following STASH to CF standard name changes:
 * ```eastward_wind``` to ```x_wind```, and
 * ```northward_wind``` to ```y_wind```.
This causes some subsequent CML cube content reordering.

This PR should be rebased once #841 is merged.

Note that:
 * The ```LBFC_TO_CF``` UM/CF map now no longer contains the invalid translation mappings for fieldcodes **981**, **982** and **984**.