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.
Do we really need a global here? It feels a bit C-ish...