drizzle pixscale adjustment
  * in the second (final) drizzle pass, the drizzle parameters
  (pixscale and pixfrac) are now set with the assumption of 2 exposures per
  epoch.  This ensures that we get consistent default pixel scales across all
  epochs, regardless of the  varying number of exposures per epoch.
bug fixes for refim construction
  * cleaned up the definition of the deepest visit when defining the refimage
  (i.e. when the user provides no --refvisit explicitly)
  * now the refimage flt files are copied directly from fltdir into the
  refim directory (instead of copying them from the epoch directories)