NAME:
  altaz
 PURPOSE:   (one line only)
  Compute altitude and azimuth on sky given equatorial coordinates
 DESCRIPTION:
 CATEGORY:
  Astronomy
 CALLING SEQUENCE:
  altaz,ha,lat,dec,alt,az
 INPUTS:
  ha - hour angle of position on sky (radians)
  lat - Latitude of observing vantage point (radians)
  dec - Declination of position on sky (radians)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  alt - Altitude of position above horizon (radians)
  az  - Azimuth of position (radians)
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2010/07/02, Written by Marc W. Buie, Southwest Research Institute.  Ported
                from altaz.c in my C-library.