added --singlesubs
 * with --singlesubs flag, you will get _single_sci.fits drizzled images
   and the corresponding _wht and _bpx products at the driz2 stage.
 * You also get single_sub_sci.fits files from the --diff stage
 * Note that the single_sci file names are distinguished by including
   the full rootname from the file header, which is unique to each
   exposure.   Its a bit of overkill, perhaps, but it does the job.
 * Oh, and the _masked.fits files are also produced for each single_sci.