Fix lbfc mapping rules

Fixes rule deriving lbfc from CF name+units; includes new "um_cf_map.py" from updated metOcean definitions; remove older rules this replaces.

This requires https://github.com/metarelate/metOcean/pull/7 to be merged first.

Prerequisites:
- [ ] check metOcean PR completed https://github.com/metarelate/metOcean/pull/7
- [ ] recheck that new 'um_cf_map.py' matches latest metOcean content, via latest iris-code-generators