[InverseDynamics] Support for Jacobians & derivatives

This change adds support for calculating Jacobians and
dot(Jacobian)*u terms, along with the required support for
the 3xN matrices in the standalone math library.
It also adds functions to compute kinematics only (position, velocity, accel).
To facilitate tests, the Cl also adds a RandomTreeCreator to create
randomized multibody trees.
Thanks to Thomas Buschmann for this contribution!

Extras/InverseDynamics/invdyn_bullet_comparison.hpp

@@ -1,12 +1,13 @@
 #ifndef INVDYN_BULLET_COMPARISON_HPP
 #define INVDYN_BULLET_COMPARISON_HPP
 
+#include "BulletInverseDynamics/IDConfig.hpp"
+
 class btMultiBody;
 class btVector3;
 
 namespace btInverseDynamics {
 class MultiBodyTree;
-class vecx;
 
 /// this function compares the forward dynamics computations implemented in btMultiBody to
 /// the inverse dynamics implementation in MultiBodyTree. This is done in three steps