Multiprocessing option for first guess water vapor retrieval.
Using only 1 CPU, the first guess water vapor retrieval may take up to 10 minutes for a full EnMAP scene. This merge request implements a multiprocessing option, which significantly decreases the processing time, e.g., to about 1 minute using 16 CPUs.
Signed-off-by: nbohn nbohn@gfz-potsdam.de