Find the other diameter of an ellipse given its circumference and one of its diameters.

Calculates one radius of an ellipse (either the major or minor axis) given the circumference and one of its radii.
*The circumference of an ellipse is given by the major axis radius a, the eccentricity k = sqrt(1 - b^2/a^2), and the second kind of complete elliptic integral E(k), as follows:
L = 4a E(k)
.The Newton-Raphson method is used to perform this inverse calculation.