bezier curves

I think a good way to go about this would be to add a `Knot` type that inherits from `Keyframed`, and then the actual bezier curve reduces to a specialized interpolation method. The `Knot` would carry `Handle` vectors, which the interpolator would expect to find on the knot keyframe.

I'm pretty sure this would also permit mixing bezier interpolation with other interpolation methods, it would just require that the keyframes for the bezier interpolation carry the appropriate supplementary information (handles)