Takes a reflectivity (default=1) as input and reflects rays in a ideal geometry
parabolic mirror. The mirror is positioned in the zx-plane curving towards positive y.
I.e. the focal point is (0,0,f(a,b))
Parabola is y = z^2 / a^2 + x^2 / b^2
the focal length for the 'z' curve is f=a^2 / 4
Parameters in boldface are required;
the others are optional.