add real-time safe Inverse Dynamics library+test+utils

an example for the example browser will follow.
thanks to Thomas Buschmann

Extras/InverseDynamics/SimpleTreeCreator.hpp

@@ -0,0 +1,34 @@
+#ifndef SIMPLETREECREATOR_HPP_
+#define SIMPLETREECREATOR_HPP_
+
+#include "MultiBodyTreeCreator.hpp"
+
+namespace btInverseDynamics {
+
+/// minimal "tree" (chain)
+class SimpleTreeCreator : public MultiBodyTreeCreator {
+public:
+    /// ctor
+    /// @param dim number of bodies
+    SimpleTreeCreator(int dim);
+    // dtor
+    ~SimpleTreeCreator() {}
+    ///\copydoc MultiBodyTreeCreator::getNumBodies
+    int getNumBodies(int* num_bodies) const;
+    ///\copydoc MultiBodyTreeCreator::getBody
+    int getBody(const int body_index, int* parent_index, JointType* joint_type,
+                vec3* parent_r_parent_body_ref, mat33* body_T_parent_ref, vec3* body_axis_of_motion,
+                idScalar* mass, vec3* body_r_body_com, mat33* body_I_body, int* user_int,
+                void** user_ptr) const;
+
+private:
+    int m_num_bodies;
+    idScalar m_mass;
+    mat33 m_body_T_parent_ref;
+    vec3 m_parent_r_parent_body_ref;
+    vec3 m_body_r_body_com;
+    mat33 m_body_I_body;
+    vec3 m_axis;
+};
+}
+#endif  // SIMPLETREECREATOR_HPP_