* [Py] Using C symbolic library and Python user is possible even with external forces.
* [Py] Adding missing fields in MbsData.
*[dirdyn][Fix] the function user_dirdyn_loop is now called only at the real time steps, even for multisteps integration methods.
*[dirdyn][Fix] the mbs_data->flag_stop is taken into account in all the integrators now, the simulation stops.
*[integrators][dirdyn] Add a new integrator called 'AlphaM' for alpha method. It is explicit, fixed step-size but with two stages and a bit smarter than Euler explicit :-)
