Subroutine:	IRCO_PROTWIST

 Purpose:	Project points and twist angles onto a plane.

 File: 	irproject.shl.

 Author:	Do Kester

 Use:
	call IRCO_PROTWIST(
		prid,	I	integer,
 		xyz,	I	doubleprecision array (3,N),
 		twv,	I/S	doubleprecision array (3,N),
  		x, 	O	doubleprecision array (n),
  		y,	O	doubleprecision array (n),
  		t,	O	doubleprecision array (n),
		n 	I	integer.
		)
	prid	a projections system number (see IRCO_PRNAME),
	xyz	3-dimensional coordinates of the points on the sphere,
	twv	3-dimensional coordinates of the twistangles on the sphere,
	x, y	the corresponding coordinates on the plane,
	t	the projected twist angles on the plane
	n	number of points to be projected.
 Description:
     	The xyz-vectors are projected using IRCO_PROJECT.
     	To find the projected twist angles a small fraction of the
	twist angle vector (length: 3.85 arcmin) is added to the
	xyz-vector to simulate the shift in xyz during one second.
	The resulting vector is projected too and the twist angle
	is calculated from the difference between the original
	and the shifted projections.

	The twv-vector is used as scartch space; it is destroyed.

 References:
	IRCPO_PROJECT IRCO_PRNAME.
 Updates: