NAME:
  elem2xyz
 PURPOSE:   (one line only)
  Compute position given orbital elements
 DESCRIPTION:
 CATEGORY:
  Celestial Mechanics
 CALLING SEQUENCE:
 INPUTS:
  m    - Mean anomaly (radians)
  arg  - Argument of perihelion (radians)
  node - Longitude of ascending node (radians)
  inc  - Inclination (radians)
  ecc  - Eccentricity
  a    - Semi-major axis (AU)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  JDEPOCH - Julian date of mean anomaly (default=0.0, ie., doesn't matter)
  JDPOS   - Julian date desired (default=JDEPOCH)
 OUTPUTS:
  x    - Heliocentric x coordinates (AU)
  y    - Heliocentric y coordinates (AU)
  z    - Heliocentric z coordinates (AU)
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2002/05/29 - Marc W. Buie, Lowell Observatory.  Ported from fortran routines
    supplied by Larry Wasserman and Ted Bowell.