FUNCTION: sum3d PURPOSE: Takes two 3D structures and returns a single 3D structure whose data is the sum of the two INPUTS: d1,d2 each must be 3D structures obtained from the get_?? routines e.g. "get_el" RETURNS: single 3D structure CREATED BY: Davin Larson LAST MODIFICATION: @(#)sum3d.pro 1.8 02/04/17 Notes: This is a very crude subroutine. Use at your own risk.