Add detmask

>> dso = add_detmask(dso,detector_mask_file)

• IXTdata_source operation
• dso is an IXTdata_source
• detector_mask_file is a string defining the location of the detector mask file to be loaded, eg. 'X:\maskfiles\inst.mask' or 'inst_masks:::myfile.msk'
• this will add a detector mask file location to an IXTdata_source object