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.
The one thing that I thought required further testing might be `_optimal_dimensioning_structure` permutations that reflect how this function give its answer.  `test_1d_duplicates_order` tests this somewhat.  It maybe that the behaviour of `_optimal_dimensioning_structure` is reflected in your tests, however this would need to be described in the comments for your tests since this is a hidden codepathway.