MetOcean UM/CF mapping translations.

This PR replaces #13 and introduces the capability to auto-generate Python dictionary encodings of metOcean CF/UM mapping translations. These encoded translations can then be injested by Iris to support its concept translation mechanism in a controlled and well-defined manner.

These current translations supported in this PR are:
    * **UM stash-code** to **CF standard name, long name, and units**,
    * **UM field-code** to **CF standard name, long name, and units**, and
    * **CF standard name, long name, and units** to **UM field-code**.