UM fields array structure identification for fast loading.

Context: 
The key external interface is "group_structured_fields"  (in iris.fileformats.um._fast_load_structured_fields).
 * this returns 'FieldCollation'-s ..
 * .. which are in effect triples of (vector_dims, dimension_elements, element_arrays_and_dims)
 * .. so something else can turn that information into cubes.

There is no actual user API yet. 
Most probably will be an experimental 'iris.load'-like function, which will take filenames + return cubes.
There's only one commit in this PR right now, so it should be simple to clear this up:
 1. Edit `lib/iris/experimental/fileformats/__init__.py` and set the `4` back to a `3`.
 1. `git add lib/iris/experimental/fileformats/__init__.py`
 1. `git commit --amend`
 1. `git push origin +pp_collate`