Use ADOL-C for derivatives of the rod energy

Previously, the rod3d code used a hand-implemented first derivative
of the rod energy, and FD for the second derivatives.  This patch
replaces this by ADOL-C for both first and second derivatives.
Advantages are:
* The code is much shorter, and easier to understand.
* The previous code contained interpolation formulae for
  first-order 1d geodesic finite elements.  This restricted
  rod problems to first-order GFE.  A follow-up patch will
  allow higher-order approximations.
* The new code runs roughly twice as fast.