NAME:
  parseop
 PURPOSE:   (one line only)
  Parse an operation line from photometry reduction, reduc.inf, file
 DESCRIPTION:
 CATEGORY:
  Photometry
 CALLING SEQUENCE:
  parseop,str,info
 INPUTS:
  str - A single string with information to parse
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  info - Structure with information pulled from the string.
           This structure will always exist with at least one
           tag, info.error.  This will indicate if the parsing
           was successful.  A successfully parse operation string
           will have all of these tags:
             op    - the name of the operation
             nvals - the number of values found for the operation
             vals  - the values found for the operation
             error - Flag, set if there is an error with the line
                       zero if the line is parsed successfully
             ok    - Operation is flagged as "ok", this is used to
                       tell the calling program that this rule has
                       already been run and probably doesn't need it
                       again.
           The tags above are always relevant.  The tags below are
           always processed but may or may not be relevant for a
           given operation.
             k2    - second order extinction and error (2-element array)
             ct    - color term and error (2-element array)
             k     - extinction and error (2-element array)
             c2    - color^2 term and error (2-element array)
             x2    - airmass^2 term and error (2-element array)
             kt    - Flag, if set requests time-dependent extinction term
             td    - String, rundate of night to use transformation from
             rc    - Reference color
             rx    - Reference airmass
           All of these are given sensible default values (0, false,
           empty string) as dictated by the variable.
           
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2014/03/06
  2018/03/12, MWB, added support for fixed reference airmass and color