NAME:
   initvc
 PURPOSE:
   Compute unit vectors for a spherical surface map.
 DESCRIPTION:
 CATEGORY:
   Image display
 CALLING SEQUENCE:
   initvc,nx,ny,xn,yn,zn
 INPUTS:
   nx  - Size of map in x direction (same as longitude)
   ny  - Size of map in y direction (same as latitude)
         If nx & ny are non-zero, the map is assumed to be a rectangular
         projection map.  If ny is zero, then nx is the total number of
         annuli and the "optimal" tiling scheme is assumed.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
   xn,yn,zn - Unit vector for each tile in the map.  These will have the
              same dimensionality as the map, ie., nx by ny or just nx.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
   Written by Marc W. Buie, Lowell Observatory, 1998 Sep 7
   2003/09/18, MWB, added rectangular map option code