Subroutine:   IRCO_TRANSMAT
 
 Purpose:      defines transformation matrix from Euler angles
 
 Category	WORLD COORDINATES, IRAS

 File:         irco.shl
 
 Author:       Do Kester
 
 Person resp.: Do Kester
 
 Address:      guspace!do or rugfx4!do         (uucp)
 
 Use: call irco_transmat(
 	zold,		I	double precision
 	theta,		I	double precision
 	znew,		I	double precision
 	mat )		O	double precision array(3,3)
 
 Description:  
 MAT is a right-handed rotation matrix with its z-axis directing to
 the north pole and its x-axis pointing to longitude = 0.
 90 - ZOLD is the longitude of the ascending node in the old system
   oldlong = 90 - ZOLD => ZOLD = 90 - oldlong
 THETA is the angle between the poles of the old and the new system
 90 + ZNEW is the longitude of the ascending node in the new system
   newlong = 90 + ZNEW => ZNEW = newlong - 90
 The rotation matrix follows the description in Lieske etal. 1977
 
 Externals:    
 
 Updates:      5 jan 1990 DK, documentation only