Re-instate CFA-netCDF functionality

[CFA-netCDF](https://github.com/NCAS-CMS/cfa-conventions/blob/master/source/cfa.md) was functionality was removed at cf version 3.14.0, with the promise of it being re-instated soon for version CFA-0.6.

Once this is done, CFA-0.4 will not be supported, so any CFA-netCDF files created using CF versions prior to 3.14.0 will need cf version 3,13,1 (or earlier) to read them.