Bugfix for singlestar drizzling.
* I was using the iminmed algorithm when combining single-epoch stacks
  and imedian for the multi-epoch stacks.   It turns out that this
  leads to a bias in the flux at the core of a bright star, because the
  poisson noise that spikes upward gets rejected as a Cosmic Ray and
  triggers the use of the median value -- which is abnormally low
  due to the high poisson noise at the center of a bright star.
* the bias was really strong only for the single-epoch driz2 products
* I've adjusted the drizzle settings to make a more intelligent choice
  for combine_type, and to set the combine_nhigh rejection number
  based on the number of contributing FLTs.