minor bug fixes and adjustments
exposures.py + sndrizpipe.py
  * use the pidvisit instead of visit for exp sorting and nat_drz_sci naming,
    in order to avoid potential conflicts from two visits in
    the same epoch that happen to have the same visit name, but
    are from two different programs.
drizzle.py :
  * even with --drizcr on, don't erase CR bit flags if we only have 1 exp
major changes in refimage construction and use

improvements :
  * refimage is now built using firstDrizzle (full frame, native rotation,
  auto-selecting the optimal pixscale and pixfrac based on number of flts)

  * then we run tweakreg on the drz product, instead of on the list of flts

  * added getpixscale() to register.py, so tweakreg conv_width is set
  appropriately for the input image pixel scale

bugfix :
  * turns out that tweakreg chokes if the input ref catalog has only RA,DEC
  columns, but the TweakReg call includes a colname for the flux units.
  This is now corrected in register.py by checking the catalog.