@aconley  - I think it's probably better to just require it at this point.  The whole point of `Quantity` is to make it so that code no longer has to guess what unit the user is asking for.  I'm fine leaving `Tcmb0` as a grandfathered-in backwards compatibility item (although a deprecation warning would be nice there), but for new stuff the guiding principal is that we require units in this way from now on.  If nothing else, this will *force* interested users to learn enough about `Quantity` to realize it lets them easily specify their units, although from that perspective there should be an example of its usage here.