I think this name is a bit confusing, because it implies it's a `value`, which for `Quantity` means *not* a quantity.  Perhaps this should instead be `functional_quantity`?  

Better yet, could this be changed to something a bit less ambiguous for someone not familiar with the package? For `Magnitude`, something like `mag_quantity` would be ideal. But I see the problem that that's intrinsic to the subclass, and here you want something general for all functional quantities.  So instead it might be better to make this and the current `quantity` property clearly related: e.g., `physical_quantity` and `functional_quantity`, or `input_quantity` and `output_quantity`?  I admit those aren't perfect either, but perhaps someone else has a better idea?  The general point is that I think `functional_quantity` would be very mysterious for a user who encountered it while just reading someone's code...