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.
I think this is a declaration of intent rather than a real requirement.  
I don't know if an actual error might occur at some point, and I'm not really sure it's important.
I imported this comment wholesale and TBH it probably needs a revisit -- will do that.