write documentation about how to add proxy variables to pyaerocom

[jgriesfeller]

Describe a problem and/or suggest an improvement
It's not clear currently how to add additional non observed variables to pyaerocom. Describe how that is done.

Additional context
citation from feedback at AeroTools user meeting:
How can one expand the list of non-observable variables ? Is there a recipe? Eg speciated dry and wet deposition, atmospheric burden, surface concentrations, temperature, precipitation, radiative forcing, anthropogenic aod…can we have global means as a table eg in heat maps

[jgriesfeller]

Regarding the global means in heat maps: There was a configuration option for that used in task4041 of CAMS2-40:

CFG["model_only_stats"] = True

that did exactly that. I'm not sure if that still works after the recent refactoring.

[lewisblake]

This has to do with how variables are displayed on the maps. The renaming which occurs on the main web page does not seem to be applied to the maps pages.

With regards to adding custom variables: @dulte can you please add some documentation to the readthedocs on how to do this? It looks like in the original PR this was not addressed. #1147 If documentation does exist, please link here.