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.
A contextual example would be helpful here. Perhaps (please edit as desired):

```
For example, a FieldCollection could represent the 43200 2-dimensional fields for a single phenomenon which represents 50 vertical levels, 24 ensembles and 36 timesteps. The puprose of this class to identify the structure from the iterable of fields.

```