Add a Plane object

[bfoz]

That infinite flat thing that divides a space into two half-spaces. It can be specified with an origin and a surface normal vector, or a surface normal vector and a distance from the origin along normal.

Make shortcut methods for creating Planes with axis-aligned normal vectors.

The normal vector points to the outside of the plane. The distinction becomes important for CSG and clipping, so I'm specifying the convention up front.