Implement regulators for the Euclidean coordinates of `Point` entities, automatically creating all three of them for each added point entity. When such a regulator is set, it freezes the corresponding representation coordinate to the set point. In addition, if all three coordinates of a given `Point` are set, the coradius coordinate (which holds the norm of the point) is frozen as well.
Note that a `PointCoordinateRegulator` must be created with a `Point` as the subject. This commit modifies `HalfCurvatureRegulator` analogously, so that it can only be created with a `Sphere`.
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>
