add GCOS quality criterion to the statistics

[jgriesfeller]

In the old aerocom interface there's the calculation of the GCOS envelope for total AOD and fine mode AOD (the green line in this plot). For C3S_Aersol we are obliged to provide these numbers as well, but they are right now not in the statistics calculation. The criterion is depending on the variable. A table for aerosols can be found at the WMO. They also exist for other variables like Ozone or Methane
I will implement that, but the question is (due to the variable dependency) where exactly to do that.

[jgliss]

@jgriesfeller how exactly is the calculation done? Do you have a formula? And where in your processing chain would you need these envelope? In AeroVal? Or for ColocatedData.calc_statistics?

In any case, can you please branch off main-dev to implement the updates?

[jgriesfeller]

A link to the newest GCOS definitions:
https://library.wmo.int/index.php?lvl=notice_display&id=22135#.ZFpJs5FBxAs

[lewisblake]

@jgriesfeller Is this still needed by any project?