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

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

Extras/InverseDynamics/btMultiBodyTreeCreator.hpp

@@ -0,0 +1,50 @@
+#ifndef BTMULTIBODYTREECREATOR_HPP_
+#define BTMULTIBODYTREECREATOR_HPP_
+
+#include <vector>
+
+#include "BulletInverseDynamics/IDConfig.hpp"
+#include "MultiBodyTreeCreator.hpp"
+#include "BulletDynamics/Featherstone/btMultiBody.h"
+
+namespace btInverseDynamics {
+
+/// MultiBodyTreeCreator implementation for converting
+/// a btMultiBody forward dynamics model into a MultiBodyTree inverse dynamics model
+class btMultiBodyTreeCreator : public MultiBodyTreeCreator {
+public:
+    /// ctor
+    btMultiBodyTreeCreator();
+    /// dtor
+    ~btMultiBodyTreeCreator() {}
+    /// extract model data from a btMultiBody
+    /// @param btmb pointer to btMultiBody to convert
+    /// @param verbose if true, some information is printed
+    /// @return -1 on error, 0 on success
+    int createFromBtMultiBody(const btMultiBody *btmb, const bool verbose = false);
+    /// \copydoc MultiBodyTreeCreator::getNumBodies
+    int getNumBodies(int *num_bodies) const;
+    ///\copydoc MultiBodyTreeCreator::getBody
+    int getBody(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:
+    // internal struct holding data extracted from btMultiBody
+    struct LinkData {
+        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;
+    };
+    std::vector<LinkData> m_data;
+    bool m_initialized;
+};
+}
+
+#endif  // BTMULTIBODYTREECREATOR_HPP_