Fix lbfc mapping rules

Addresses: https://github.com/SciTools/iris/issues/964

Fixes rule deriving lbfc from CF name+units; includes new "um_cf_map.py" from updated metOcean definitions; remove older rules this replaces.
Mostly wanted to simplify, but also because the existing rules do not check the units which is obviously dubious.

**This requires https://github.com/metarelate/metOcean/pull/7 to be merged first.**

Prerequisites:
- [x] check metOcean PR completed https://github.com/metarelate/metOcean/pull/7
- [x] recheck that new 'um_cf_map.py' matches latest metOcean content, via latest iris-code-generators

Reviewer: cpelley