[Fix] warning or error use `used unitialized variable` mbs_data->process in userjointforce when doing equilibrium.

comments of custom integrator and integrating option for close kin at each time steps : to be continued

This mainly consist in calling the MbsWorld3D constructor with the 2 new
arguments, i.e. the WindowManager object and the WorldRenderer object.

New folder for project to avoid user to be confused between his own MBproject folder and the one from the sources