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
never mind -- I see that you are just solving an equation here. What you did is right.