Computes field mean value and field standard deviation using CDO operators fldmean and fldstd, and discarding lat/lon coordinates from CDO output
References : https://code.zmaw.de/projects/cdo/embedded/1.6.4/cdo.html#x1-3040002.8.5
Provider / contact : climaf at meteo dot fr
Mandatory arguments:
Climaf call example
>>> ds= .... #some dataset, with whatever variable
>>> m=mean_and_std(ds) # m receives the field mean
>>> dev=m.std # dev receives operator output named "std", namely the field standard deviation
Side effects : create temporary files in the directory provided for output fields
Implementation : just two calls to cdo and the use of ncwa for discarding degenerated space dimensions (because CDO does not discard them)