[RobotUtil] Use RobotUtilsShrPtr

This partially fixes #60 (closed) by using the shared pointer for RobotUtil instead of a raw pointer. I haven't run anything, but it is compiling with sot-core 4.2.0 and I do not see why it should not work. As for the parameter server, I noticed a small problem concerning the way RobotUtil is loaded (specifically, the value of m_robot_util->m_nbJoints). We can discuss about it, for now we keep using our own ParameterServer