iris.fileformats.um is designed as a replacement for the older iris.fileformats.ff.
However, it currently just wraps the old code.
In principle, we would like a solution based on 'mule' instead

This also applies to the _fast_load_structured_fields code : At present that is private, and only used by iris.experimental.fieldsfile, but we want to make it public : See #2083
"Make structured FF loading public"

Mule : see https://code.metoffice.gov.uk/trac/um/browser/mule
(requires account)