NAME:
    gxpar
 PURPOSE: (one line)
    General purpose FITS file header keyword extraction.
 DESCRIPTION:
   This procedure is a 'front-end' to SXPAR and may be used to extract the
 value of a keyword from a FITS file header.
   Because there is so much variation of the keywords found in FITS file
 headers, the idea is to have and use a set of 'standard' keywords in
 programs that load and display FITS files.
   To facilitate this, gxpar uses a correspondence list to associate a
 standard keyword with a keyword found in a given FITS file header (or with
 a value different from that which might be in the header).
   This correspondence list consists of three linear arrays of the same
 length.  The first array contains standard keywords, the second contains
 flags which indicate the type of correspondence, and the third contains
 the items corresponding to the standard names in the first array:

           stdkeywords   flags   hdrkeywords

   The flags are single letters from the set 'K', 'D', 'F', 'L', or 'S',
 indicating a keyword, floating, double, longword, or string, respectively.
 Flags having the letter 'K' associate standard keywords with header keywords.
 All other flag letters associate standard keywords with 'direct' values.

   Standard keywords in use are:
           airmass
           date
           exptime
           filter
           jd
           mjd
           object
           time

   Examples:
      airmass k  airmass
      airmass f  1.73
      filter  k  filtnam1
      filter  s  f555w

   Note: This procedure treats the correspondence list as case-insensitive,
 except for the third field of a record having a flag of 'S'.

 CATEGORY:
    File I/O
 CALLING SEQUENCE:
    gxpar, header, keyword, stdkeywords, flags, hdrkeywords, value, error
 INPUTS:
    header      : A FITS file header array (as returned from READFITS).
    keyword     : The standard keyword whose value is to be obtained.
    stdkeywords : List of standard keywords (first correspondence field).
    flags       : List of flags (second correspondence field).
    hdrkeywords : List of header keywords (and values) (third correspondence
                  field).
 OPTIONAL INPUT PARAMETERS:

 KEYWORD PARAMETERS:
    SILENT = If set, suppresses all message output.
 OUTPUTS:
    value : The returned value.
    error : Non-zero, if an error is encountered.  Incremented on each call
            which detects an error.  The caller should set this to zero before
            any sequence of calls to this procedure.
 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  Written by Doug Loucks, Lowell Observatory, November, 1993.
  2000/11/08, MWB, removed use of !err