Added the ability to get the radius of a curve at its midpoint in aprepro.  The command is Radius(id), where id is the ID
of the curve.  For example, 

brick x {Radius(3)} 

will create a brick whose size is the radius of curve 3.  If the curve is straight or otherwise has infinite radius at
its midpoint, or if the curve doesn't exist, a radius of 0 will be returned.

Since the radius at the midpoint is used, this command will probably only be useful for circles.

Satisfies bug request #752.

- Darryl Melander