Add @quantity decorator

After adding [this](https://github.com/ufo-kit/concert/blob/8656a648a98c1cdc67cf87fb60e3649f66b379a8/concert/devices/cameras/uca.py#L246-L252) property to the `camera.Ufo` class, I noticed that I actually wanted to specify an additional quantity rather than just a stupid property.

This PR adds a `quantity`decorator that can be applied to a method that returns the corresponding value. Initially, I wanted to make this part of the `Quantity` class but it turned out that it's not easily doable.

One thing to remember: this will never replace `Quantity` because we still want to provide a consistent API for base devices. The `quantity` decorator should only be used for quantities that are not common to all devices of a certain base class (e.g. the `clock_cycles` of the Ufo camera).