fixed an import error, disabled flux corrections.
  * added the cntrd.py module (from IDL's astrolib via Dave Jones)
    to compute the center position of the psf stamp.
  * this replaces a module from the old hstsnpipe/tools/hstsnphot.py
    that was not included in this package.
  * Also : disabled the flux correction factor, which requires
    predefined zero points and aperture corrections for any
    filter to be used.  COuld be resurrected later if it turns
   out that the output PSF flux values are not matching the input.