public class DHLink
A link class to encapsulate the D-H parameters for a given link in a robotic
d - the "depth" along the previous joint's z axis theta - the rotation about
the previous z (the angle between the common normal and the previous x axis)
r - the radius of the new origin about the previous z (the length of the
common normal) alpha - the rotation about the new x axis (the common normal)
to align the old z to the new z.