Add FieldCollation -> metadata conversion.

Adds iris.experimental.fieldsfile.convert_collation().
:bomb: :fire: :ambulance: :fire_engine: :police_car:  ... 

You need to apply the first `DimCoord` past the post into the `dim_coords_and_dims` bucket as per the `_convert_vertical_coords`, and all others into the `aux_coords_and_dims` bucket. 

When integration testing (manually), I'm seeing *two* dimensional coordinates (`forecast_period` and `time`) trying to bind to the same dimension, causing the cube construction to riot ...