Return Maybe values from Arc2d.centerPoint etc.

[ianmackenzie]

It would be safest for functions like Arc2d.centerPoint, Arc2d.radius and corresponding functions in Arc3d (including 3D-specific ones like Arc3d.normalAxis) to return Maybe values when the arc swept angle is zero (the arc is actually a straight line). Currently they just return infinite values which are not super useful, and it's easy to forget or not realize that this is the case.