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
not sure, but don't you elsewhere allow the user to pass points to be used and then do something where you use the given symbols or else create dummies if symbols are not passed?, e.g. 

```
def perpendicular_line(self, pt, *xyz):
    x, y, z = xyz if xyz else [Dummy(i) for i in 'xyz']
```

Something to that effect. See other similar methods.