NAME:
  mkhull
 PURPOSE:   (one line only)
  Create a smoothed outline based on an occultation limb profile
 DESCRIPTION:
  This routine attempts to create a smooth outline that passes through a set
    of occultation limb points.  It is not meant to be a fit but can still
    be a useful tool rather than fitting strict mathematical shapes.
 CATEGORY:
  Occultations
 CALLING SEQUENCE:
  mkhull,xi,eta,hxi,heta
 INPUTS:
  xi - Tangent plane coordinate of measured point on the limb
  eta - Tangent plane coordinate of measured point on the limb
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  NPTS - Number of points in the outline (default=100)
  SIGMA - spline control factor (see spline.pro documentation), default=0.5
 OUTPUTS:
  hxi - coordinates on the smoothed outline
  heta - coordinates on the smoothed outline
 KEYWORD OUTPUT PARAMETERS:
 CONFIGURATION:
 COMMON BLOCKS:
 SIDE EFFECTS:
 PROCEDURE:
  The Cartesian coordinates are converted to polar coordinates, sorted by
    angle, padded by 2pi before and after the data with a copy of the data,
    spline fitted, then the principal value is converted back to Cartesian
    coordinates.
 RESTRICTIONS:
  This won't work if there aren't enough points but where, when, and how
    it will fail isn't easy to predict.  The coordinate origin should
    be interior to the hull and works best if the origin is actually in
    center (though this isn't strictly required).
 MODIFICATION HISTORY:
  Written by Marc W. Buie, Southwest Research Institute, 2023/03/25