Added more Tests to the Plane class.

- [x] change Plane.equal to Plane.is_coplanar
- [x] change Plane.are_coplanar to handle Point3D, LinearEntity and Plane as arguments; the logic of Plane.equal should be there
- [x] change Point3D.are_coplanar to Point3D.is_coplanar
- [x] optionally add a function to utility like `are_similar` that is named `are_coplanar` if you think that's needed
Do you think we should show at least one case of showing, perhaps in the next example, ` Line3D.are_concurrent(l2, l3, l4)`?