Metarelate Types

Gen translations is updated to use the newly released Metarelate API.

This reproduces 
iris.fileformats.um_cf_map 
and 
iris.fileformats.grib._grib_cf_map

with one exception: a mistake in um_cf_map caused a line to be written twice, which should not occur; this pull request removes that repeat line
I'm not sure where `CFFieldcodeMapping` comes from. Then I saw ```from translator import *```. I'm not a fan of this approach at all, but it's not related to this PR.