Make explicit that body fixed frame is the sourceFrame

As the frames are formally described as sourceFrame and targetFrame,
make it clear that the angular velocity is represented in sourceFrame.
Let an indication that it is the "body" frame, as in the RigidBodyState documentation

src/samples/RigidBodyState.hpp

@@ -61,7 +61,7 @@ namespace base { namespace samples {
 	 */
         base::Matrix3d cov_velocity;
 
-        /** Angular Velocity as an axis-angle representation in body fixed frame
+        /** Angular Velocity as an axis-angle representation in sourceFrame (body fixed frame)
          *
          * The direction of the vector is the axis, its length the speed */
         base::Vector3d angular_velocity;