Add SeparationPlane volume object

While trying to create a flat-Earth simulation, I got an error that the curved leapfrog algorithm does not have an implementation for intersect with Box objects. The correct way to implement this would be to check all 6 sides of the box for an intersection, which will be really slow.

So instead I simply implemented a volume, SeparationPlane, which is defined as being on one half of a Plane. This will simplify the tracking algorithms as well as simplifying the construction of the universe.