Change DegreesOfFreedom to take Points instead of Vectors

the `typedef Vector<Length, ICRFJ2000EclipticFrame> Position;` should now define `Displacement`.
You can add a `typedef Point<Displacement> Position`.
Similarly for `Velocity` (I can't find a good name for the equivalent of `Displacement` though, the only ones I came up with were `RelativeVelocity` and `VelocityOffset`).