NAME:
  hstextract
 PURPOSE:   (one line only)
  Define target position and extract sub-frame for processing
 DESCRIPTION:
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  hstextract,filestem
 INPUTS:
  filestem - String that is used to select a set of file for processing.
             The search string for files will be
               filestem+'*.flc.fits.gz'
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  SKIPWCS - Flag, if set skips the attempt to apply a WCS update and will
               not require it to proceed but absolute astrometry will be
               bad in this case.
 OUTPUTS:
 KEYWORD OUTPUT PARAMETERS:
 CONFIGURATION: (optional, overrides xtrack.in)
   File: config.ini
   [global]
     subdir   - naem of sub-directory  added to base path to find data
     objectid - object code (see ephem.pro)
     filetype - type of file to read ('flt' or 'flc'), default='flc'
     satur    - Saturation signal level, default=80000L
     tdw      - Fitting region half-width for stars, default=15
 objectid
   [ddir]   - section to list potential root directories to find data
     XXX - 
       XXX is a unique string (eg., D01)
        is a directory path
       for additional details see getddir.pro
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2020/06/26, Written by Marc W. Buie, Southwest Research Institute
  2020/09/24, MWB, added SKIPWCS keyword