Update CF aggregation keywords

[davidhassell]

The CF aggregation keywords have changed (cf-convention/cf-conventions#508 (comment)), and it looks like these will be the ones accepted into CF-1.13, so we need this code to understand them (and not the old ones!).

This will be largely fixed in cfdm (see NCAS-CMS/cfdm#341), and will be accessible in cfdm>=1.12.2.0