Allow a --nbright option
* when you want to select the top N brightest sources in each image and
use only that subset for alignment.
fixing nbright implementation; Added a --clean option
* fixed some bugs in the way catalogs are generated for the --nbright option
* added a new function in register.py for writing out RA,Dec catalogs from
  drizzlepac.catalogs.Catalog objects generated using the drizzlepac imagefind
  operation.

* note that --clean doesn't do much just yet.
* Still working on a --singlestar option for registering P330E