added darcsec to _epochs.txt
 * when the source RA,dec are provided, we now compute and tabulate
   the distance from the source to the target position of each image
   (usually near the center of the image)
mkscaledtemplate.py ==> pseudodiff.py
  *  expanded the remit for mkscaledtemplate.py to include diff image construction.
  * this new pseudodiff.py module takes in the target name, epoch and filters,
    then it locates the correct sndrizpipe products and makes the diff image
    including badpix masking and weight map combination.