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
encode to me is about converting something into a series of bytes, not generating code (which is a string). I'm thinking unicode here. I won't lose sleep over it.