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

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

Extras/InverseDynamics/MultiBodyTreeCreator.hpp

@@ -0,0 +1,45 @@
+#ifndef MULTI_BODY_TREE_CREATOR_HPP_
+#define MULTI_BODY_TREE_CREATOR_HPP_
+
+#include <string>
+#include <vector>
+#include "BulletInverseDynamics/IDConfig.hpp"
+#include "BulletInverseDynamics/IDMath.hpp"
+#include "BulletInverseDynamics/MultiBodyTree.hpp"
+#include "MultiBodyNameMap.hpp"
+
+namespace btInverseDynamics {
+/// Interface class for initializing a MultiBodyTree instance.
+/// Data to be provided is modeled on the URDF specification.
+/// The user can derive from this class in order to programmatically
+/// initialize a system.
+class MultiBodyTreeCreator {
+public:
+    /// the dtor
+    virtual ~MultiBodyTreeCreator() {}
+    /// Get the number of bodies in the system
+    /// @param num_bodies write number of bodies here
+    /// @return 0 on success, -1 on error
+    virtual int getNumBodies(int* num_bodies) const = 0;
+    /// Interface for accessing link mass properties.
+    /// For detailed description of data, @sa MultiBodyTree::addBody
+    /// \copydoc MultiBodyTree::addBody
+    virtual 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 = 0;
+    /// @return a pointer to a name mapping utility class, or 0x0 if not available
+    virtual const MultiBodyNameMap* getNameMap() const {return 0x0;}
+};
+
+/// Create a multibody object.
+/// @param creator an object implementing the MultiBodyTreeCreator interface
+///        that returns data defining the system
+/// @return A pointer to an allocated multibodytree instance, or
+///         0x0 if an error occured.
+MultiBodyTree* CreateMultiBodyTree(const MultiBodyTreeCreator& creator);
+}
+
+// does urdf have gravity direction ??
+
+#endif  // MULTI_BODY_TREE_CREATOR_HPP_