``Galactocentric`` and ``LSR`` now use a ``CartesianRepresentation`` instead
of ``CartesianDifferential`` for their velocity attributes (``galcen_v_sun``
and ``v_bary``, resp.).  On input, this has little effect, since instances of
both remain accepted, but for converting the attribute to a ``Quantity``, one
now has to access the ``.xyz`` attribute instead of ``.d_xyz``.
