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

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

Extras/InverseDynamics/MultiBodyTreeCreator.cpp

@@ -0,0 +1,64 @@
+#include "MultiBodyTreeCreator.hpp"
+
+namespace btInverseDynamics {
+
+MultiBodyTree* CreateMultiBodyTree(const MultiBodyTreeCreator& creator) {
+    int num_bodies;
+    int parent_index;
+    JointType joint_type;
+    vec3 body_r_parent_body_ref;
+    mat33 body_R_parent_ref;
+    vec3 body_axis_of_motion;
+    idScalar mass;
+    vec3 body_r_body_com;
+    mat33 body_I_body;
+    int user_int;
+    void* user_ptr;
+
+    MultiBodyTree* tree = new MultiBodyTree();
+    if (0x0 == tree) {
+        error_message("cannot allocate tree\n");
+        return 0x0;
+    }
+
+    // TODO: move to some policy argument
+    tree->setAcceptInvalidMassParameters(false);
+
+    // get number of bodies in the system
+    if (-1 == creator.getNumBodies(&num_bodies)) {
+        error_message("getting body indices\n");
+        delete tree;
+        return 0x0;
+    }
+
+    // get data for all bodies
+    for (int index = 0; index < num_bodies; index++) {
+        // get body parameters from user callbacks
+        if (-1 ==
+            creator.getBody(index, &parent_index, &joint_type, &body_r_parent_body_ref,
+                            &body_R_parent_ref, &body_axis_of_motion, &mass, &body_r_body_com,
+                            &body_I_body, &user_int, &user_ptr)) {
+            error_message("getting data for body %d\n", index);
+            delete tree;
+            return 0x0;
+        }
+        // add body to system
+        if (-1 ==
+            tree->addBody(index, parent_index, joint_type, body_r_parent_body_ref,
+                          body_R_parent_ref, body_axis_of_motion, mass, body_r_body_com,
+                          body_I_body, user_int, user_ptr)) {
+            error_message("adding body %d\n", index);
+            delete tree;
+            return 0x0;
+        }
+    }
+    // finalize initialization
+    if (-1 == tree->finalize()) {
+        error_message("building system\n");
+        delete tree;
+        return 0x0;
+    }
+
+    return tree;
+}
+}