Add support for aerosol optical depth climatology metrics to the AutoAssess replacement.

[catherinehardacre]

Support will be added within the Met Office's AutoAssess replacement, the Climate Assessment Workflow (CAW) for aerosol optical depth (AOD) climatology metrics . This work aims to replicate the existing AutoAssess AOD metric within the ESMValTool. The AOD metric compares modelled multi-annual seasonal means (DJF, MAM, JJA, SON) of AOD at 440 nm to a climatology developed using ground based observations from AeroNET.

This work will use updated observational AOD climatology compared to the current AutoAssess metric. However, the climatology will need to be used 'offline' because further work will be required to CMORize the station based observational data.

The work to generated a CMORized observational data set of the AOD climatologies will be done under a separate issue.

Branch and pull request
Branch for this work:
https://github.com/ESMValGroup/ESMValTool/pull/new/aeronet_aod_clim_new_diagnostic

Pull request:
#3048

[mo-abodas]

(Comment transferred from the ESMValTool workshop)
It may be worth reading this section of the CF conventions that deals with this type of geometry:
https://cfconventions.org/Data/cf-conventions/cf-conventions-1.9/cf-conventions.html#discrete-sampling-geometries

[bouweandela]

The deadline for including new work in v2.9 has passed, so I'm removing this issue from the milestone. Please add it to the next milestone if you plan to include this in the next release.