NAME:
  skymat
 PURPOSE:   (one line only)
  Compute the sky plane rotation matrix from angular coordinates
 DESCRIPTION:
 CATEGORY:
  Mathematical
 CALLING SEQUENCE:
  skymat,lat,lon,rotmat
 INPUTS:
  lat - latitude (spherical coordinate, radians, scalar)
  lon - longitude (spherical coordinate, radians, scalar)
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
  rotmat - rotation matrix (always double, 3x3 matrix)
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2011/11/29, Written by Marc W. Buie, Southwest Research Institute
                cloned from SKYMAT.F written by David J. Tholen