ENH: Add Cooling Degree Days (CDDCold{xx})

[bzah]

• icclim version: 5.2
• Python version: n/a

Description

Add Cooling degree days index.
fr: Degrés-jours de climatisation (Cumul sur la période des dépassements du seuil de > 18°C par la température qt moyenne)
en: Cooling Degree Days (Tmean > {TT}C)

In xclim it exists on xclim.atmos.cooling_degree_days.

Also, the xclim's standard_name is different from the clix-meta standard name:

• xclim: "integral_of_air_temperature_excess_wrt_time"
• clix-meta: "integral_wrt_time_of_air_temperature_excess"

According to cf, clix-meta is right. So we should also open a cross issue on xclim for that (or just a small pr).
edit: Actually xclim uses the alias version of the standard name, I missed it due to cf search not looking for aliases but it should be fixed by cf-convention/cf-convention.github.io#218

[bzah]

Implementation will be trivial once #189 is done. It will simply be an alias for icclim.generic(indicator="excess", threshold="18 degree_Celsius" var_name="tas", in_file="tas.nc")