10/9/14 Pull Request

As the diameter can always be calculated from the radius (and vice-versa), it's better to only store one of them. That way they can't get our of sync. If you store the radius, you don't need all the property magic, it can be a simple attribute, then make a property for the diameter, and calculate it on the fly when requested. In the diameter setter, you can compute the radius and set that.