New double flatfield

New method to calculate double flatfield. Processing is done on additive calculation but could be modified to match previous method if it makes things easier. Rapid median filter calculation is included in script Method by default is 2 (Vincent). Number of CPU per default is 32 (and capped to 50% of available CPU on a node). Application of the weight on the angular average is not done correctly. There are checkups to avoid redoing the jobs that were previous done but --force_overwrite is not considered at the moment for temporary files. Multi angle median is set to 0 by default as it's only necessary in some cases and is quite computing heavy.