For this class, `distance` has to be a `Quantity`, correct? If so,
```
if isinstance(distance, u.Quantity):
    if distance.unit.physical_type ...
else:
    raise TypeError("Distance should be a Quantity").
```
Alternatively, have a `unit` properly, in analogy with `CartesianRepresentation`
(though it would be a bit "unfair" that the angles would have to have their own units).