added Rect Filter

Added rect geo filter for "bounding box" filtering
I know that the other filter objects follow this same pattern of defining a "key" property. However, I really don't see a point to this because of the following reasons.

1) It is highly unlikely that this "key" will ever change. 
2) Even if it were to change, there is no reason that the users of the driver should be aware of it and therefore they shouldn't need to have the ability to set this "key" property.

Can you please remove this and maybe make it a constant in the RectangleConverter class?