NAME:
       WCS_ZPN
 PURPOSE:
       take ra & dec and turn them into x & y in ZPN coords
 CALLING SEQUENCE:
       wcs_zpn,long,lat,ast,x,y
 INPUT:
	long,lat: RA, DEC position in degrees
	ast: astrometry from header extraxted using extast
 OUTPUT:
       x,y: pixel postions using zpn projection
 PROCEDURES CALLED:
       wcs_rotate,interpol
 HISTORY:
       written by Ignas Snellen

(See /home/xmmssc/soft/idl/pro/astro/wcs_zpn.pro)