Updated badpix bit flags
  *  more aggressive badpix flagging: now warm pixels are flagged.
bug fix; added --nclip and --sigma
  * fixed a problem with passing refnbright and refcat without specifying a refim.
  * allows nclip and sigma options to be set for tweakreg from commandline and in interactive mode.
fixed source finding for tweakreg
* the source finding implementation in drizzlepac's catalogs.py is computing
the sky noise using the sqrt of 2 times the mode.  The drizzlepac manual says
it is using 1.5 times the 3-sigma-clipped standard deviation.   This latter
approach seems to work better, so I've forced it by manually computing the
skysigma in register.py, and then passing computesig=False to TweakReg.