Let frames/coordinates have (internal?) has_distance function #10294

In a fair number of places inside our coordinate machinery, we have to deal with coordinates with and without distances, and are testing this somewhat haphazardly - checking whether the representation is UnitSphericalRepresentation or whether it is Spherical and the distance is all unity. We should normalize these tests, and ideally also ensure that the second case simply doesn't happen (so that the test can be very simple).