Continue working on code related to mouse clicks.
In this update, we will prepare to implement and test code to determine whether the location
of the mouse when it was clicked was inside one of the control points or the point
at the current parameter on the curve.
To make this job easier, we are going to introduce a Circle class.
In this commit, we are just implementing the Circle class.  To do this, we implemented a
distanceTo method for Point.