Forward modeling using the
code of Won and Bevis.
For input, at a minimum, one must provide the boundary of
the model region, xmin, xmax, ymin, ymax (or depth).
more comprehensive input,
i.e. with a starting model,
will provide a model in addition:
xmin, xmax, zmin, zmax, mod, cens, n.
The mod is a list of polygons, each
with xy coordinates and a delta-rho value.