Define dimensionless_unscaled differently so it can be imported more easily

An annoyance in `Quantity` has been that one cannot import `dimensionless_unscaled` on top, since it needs the registry to be set up. By defining it instead as `CompositeUnit(1, [], [])` (as is done when one does `Unit('')`), this is avoided.  Hence, all the local imports in `quantity.py` and `quantity_helpers.py` could be removed.

Note that I need this for an upcoming PR for `MaskedQuantity` -- which in turn should help make Table/Quantity integration easier.