Added plate visualization, improved displacement curve, negative overlaps

Plate visualization is very basic right now (black squares).
Displacement curve now adds perlin noise (without any of the weird
oscillation from before) and scales it based on distance from the
fault. There's also no more hard edge to the overlap. It just smoothly
approaches zero as it moves out to the edge of the mesh. Negative
overlaps now kind of work, but I haven't generalized the algorithm for
both types (I have to specifically tweak some things for positive vs.
negative overlaps to get each to work.)
Additional changes to Plate.cpp

Not sure why this didn't just commit with the rest of them.