Consolidate `cf.read` backends with `xnetcdf`

[davidhassell]

Consolidate cf.read backends with xnetcdf (https://xnetcdf.readthedocs.io).

This is a companion to NCAS-CMS/cfdm#417 which introduces the core of the functionality, but the API changes need to propagate to cf.

A key feature is that the reading of PP and UM fields files is now handled by cfdm. This means that:

• nearly all of the code for reading these datasets can be removed (we need to retain the small amount of ode relating to Field aggregation)
• We need new code to create the 2-d latitudes and longitudes that used to be created in umread.py.