Solve for astrometric transformation from image to sky coordinates.

  x        - Image x-coordinate  (should be "normalized" to range from -1 to 1)
  y        - Image y-coordinate  (should be "normalized" to range from -1 to 1)
  xi       - Standard tanget plane coordinate (should be in arcsec)
  eta      - Standard tanget plane coordinate (should be in arcsec)
  terms    - Which fitting terms to use (see ASTTERMS.PRO)
  renormfac - Re-normalization factor for converting from normalized x,y to
                the original x,y values.
  bad      - array of flags that mark bad data on input (modified).


  EDIT - Flag, if set allows interactive bad point editing.
  MAXPASS - Maximum number of passes allowed when weeding out bad points
              during the fit.  (default=1000)
  NOAUTOCLEAN - Flag, if set suppresses all attempts to weed out bad points.
  XFLIP - Flag, if set flips x axis plot when editing bad points.
  YFLIP - Flag, if set flips y axis plot when editing bad points.
  FORCETERMS - String array, if provided, is a list of terms (from "terms"
                  input variable) that will be forced to a constant
                  value rather than being fitted.
  FORCEVAL - This can either be an array of values or a scalar string.
                If it is an array it must be a Nx2 array where N matches the
                  length of FORCETERMS and is used as the forced values in
                  the same order as FORCETERMS.  Row 0 of this array holds
                  the forced xi term values and row 1 holds the eta terms.
                If it is a string, this is taken to be the name of a file
                  in the current directory that contains the fit
                  coefficients.  This file is the same format as that
                  used for storing normal fitting coefficients (see
         for details).  However, this file uses the
                  first field to identify the name of the filter for
                  the coefficients rather than the filename as is normal
                  for this type of file.
                The default value of this keyword is 'forcecoeff.dat'.
  FORCEFILTER - String value of the name of the filter.  This is relevant
                   only when using a file for the coefficients.  If the
                   file contains information for a single filter then
                   that filter name is the default value.  However, if there
                   are two or more filters in the file then this keyword
                   must be supplied.
  SILENT - Flag, if set suppresses all (non-error) printed output messages.
                   Does not affect PROGRESSINFO

  cxi  - coefficients of xi fit.
  ceta - coefficients of eta fit.
  bad  - array of flags that mark bad data on output.

  WORSTRESID - Worst residual in "good" data in either axis (arcsec)
  XISCAT     - Scatter of xi fit (arcsec).
  ETASCAT    - Scatter of eta fit (arcsec).
  CXISIG     - Uncertainty of the xi fit coefficients.
  CETASIG    - Uncertainty of the eta fit coefficients.
  XICOVAR    - covariance matrix of the xi fit coefficients.
  ETACOVAR   - covariance matrix of the eta fit coefficients.
  PROGRESSINFO - string array with the printed information





  98/03/13, Written by Marc W. Buie, Lowell Observatory
  98/11/23, MWB, added renormfac and fixed documentation
  2000/09/14, MWB, added WORSTRESID keyword
  2001/08/27, MWB, changed to auto-scale errors with /edit
  2003/06/24, MWB, changed call to MARKDATA to use new features.
  2003/10/27, MWB, fixed subtle bug during bad point cleanup.  The test
                       was done on O-C against the std deviation of a
                       robust mean.  The test needs to remove the mean of
                       the surviving sample.
  2006/02/03, MWB, added XISCAT and ETASCAT output keywords.
  2009/07/14, MWB, added CXISIG, CETASIG, XICOVAR, ETACOVAR output keywords.
  2009/12/02, MWB, calling seqeunce CHANGED!  xiterms and etaterms
                   are now consolidated into a single input argument.
  2009/12/23, MWB, Major enhancement to add the ability to force some or
                     all of the terms.  This mode is supported by the new
                     keywords, FORCETERMS, FORCEVAL, and FORCEFILTER.
  2015/04/10, MWB, Added PROGRESSINFO output keyword