FUNCTION: sum4d PURPOSE: Takes two 4D structures and returns a single 4D structure whose data is the sum of the two INPUTS: d1,d2 each must be 4D structures obtained from the get_?? routines e.g. "get_mvn_d0" RETURNS: single 4D structure CREATED BY: McFadden LAST MODIFICATION: @(#)sum3d.pro 1.6 14/01/31 Notes: This is a very crude subroutine. Use at your own risk.