Allow climatologic standard deviation in atmosphere datasets, aggregate CHELSA-W5E5 standard deviations and add them as the only data source for now, e.g.:

hyoga.open.atmosphere(stdev='cw5e5')

The new keyword-argument should probably default to None until we have a better workflow for CHELSA-W5E5 data. At least it should default to None when the default 'chelsa' (non-W5E5) temperature source is used. I was just learning about CF cell methods, e.g. cell_methods="time: standard_deviation (interval: 1 day)". However I am not sure what the new variable standard name should be.

https://cfconventions.org/Data/cf-conventions/cf-conventions-1.11/cf-conventions.html#cell-methods