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
this should be renamed to `is_coplanar`. The sense is that it reads "is this line coplanar with that line". Although it can be used as "are these lines coplanar" that does not follow the existing convention of other "is_" methods.