Implement new features for CF-1.8
Implement new features that are a part of CF-1.8:
- netCDF4 groups
- netCDF4 string data-types
- simple geometries
Most of the work to implement these will be done externally in the cfdm library dependency: see https://github.com/NCAS-CMS/cfdm/issues for details.
Simple geometries will need some more work to allow these features to be correctly used by processing methods such as cf.Field.collapse, cf.Field.cell_area, etc.