Add Critical Soil Moisture

[nocollier]

As part of a push to develop ILAMBv3, I had implemented the critical soil moisture (CSM) metric. See code and results. We would like to port it to current ILAMB. The current version just implemented the calculations and thus is more of a model characteristic that is compared to others. Ideally we would like to compare this to reference data. Paul's cheatsheet has a good discussion of what kinds of observations could be used.

To do:

• Backport the v3 code to the v2.6
• Referenced paper was over Europe and used just summer, what do we do globally?
• One possibility is to use Fluxnet data, there are options for which variables we use.
• Fluxnet: Choose an energy variable TA (air temperature) or TS (soil temperature), the sheet suggests 'skin temperature', I am using ts from the models.
• Fluxnet: Choose a water variable SWC (soil water content). I am not sure how deep this measurement is made, I couldn't find a reference. In the calculation I use the top 10 [cm] of soil.
• Fluxnet: Choose a vegetation variable GPP? The sheet suggests ET, but Fluxnet doesn't provide this variable. Could another serve? Or should we grab ET at the sites from GLEAM? Ultimately I understand that we are only analyzing how the anomalies of each variable correlate.
• Add a score, consider a simple S = exp( - (CSM_ref - CSM_mod) / CSM_ref) )
• Consider a combination of gridded products?
• How do we organize this in the ILAMB configure file? Perhaps a new top level section called 'Coupling'?
• This index is valid for daily and monthly data. We need the model object to be able to distinguish between the two and an API for extractTimeSeries() to specify which kind of data is being requested. Move the watersheds work into ModelResult.