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 is some missing context as to what exactly we are trying to do (the documentation that does exist is perhaps at odds with what happening too).

Since @pp-mo is busy answering my comments on https://github.com/SciTools/iris/pull/1239, @rhattersley are you able to fill in the gap of @pelson's intentions with this?