Allow explicit negative distances in SphericalRepresentation - #11113

Fixes #11104, #8091

The problem was that any input Distance was turned into a Quantity instead of just passed through. It really should pass it through, as it is a subclass, so this PR allows it (which is what makes it an API change). But to fix that, a small change had to be made to Angle.__new__ to allow it to pass on keyword arguments like subok=True to Quantity.__new__.

Also, it turned out _combine_xyz was not safe for some operations with this change, but since that private function that was no longer needed anyway now our minimum numpy version is 1.17, so the first commit just turns that into a simple np.stack.