Support CF Metadata Standard Name Transformations

[bjlittle]

✨ Feature Request

Motivation

Extend the capability of iris to support the documented CF Standard Name Transformations.

For example, iris recognises the standard name tendency_of_air_temperature but not tendency_of_air_pressure_at_mean_sea_level simply because the former is defined in the CF Standard Name Table but not the later. However, the table does define air_pressure_at_mean_sea_level (alias for air_pressure_at_sea_level).

Ping @abooton