3-D Kirchhoff migration code:

The main concern is how to manage the data structure EFFICIENTLY.

 -. Conversion from 2-D code. 6/10/1997

 -. Add a scheme for determining n1box, n2box, and n3box
    incorporating the parameter "margin".  By controlling the
    parameter, we may save computation time and memory. 6/16/97

 -. Adjust the Maissa's code "ttsolve3d.f" so that bug-out print is
    reasonably simplified.  6/17/97

