Fix the creation of the summary SPT file

[mdlpstsci]

When processing an association for which a PROD-DTH is to be an output product, CALACS is supposed to loop over the list of raw files (where it is assumed there is an existing corresponding SPT file) and combine all the individual SPT images into one combined file. The software uses the first file in the list to create the primary header of the new output SPT. However, if the first SPT file is missing, no output SPT is generated and CALACS aborts. Further, if there are multiple SPT images in a single SPT file, all the images should be inserted into the new output file, but this is not done.