#3 allow orgin to be changed

[Toby Donaldson]

Location (0,0) in the TheMatrix is currently always the
top left corner of the frame. This is the standard
location in graphics.

However, in mathematics, it is often convenient to make
(0,0) be in the middle of the screen, or the bottom
left corner.

At a "setOrigin(x,y)" and "setOrigin(Point p)" feature
that sets the origin to be at (x, y). By default, it
should be at (0, 0) so that previous code continues to
work correctly.