Major change in the interface to LCP solver technology.
Previously, solvers had a boolean flag in Solve(..) to ask that the [M]*v_old  term was added to the known term (forces, etc.) .
Now this is not needed anymore, because the LCPprepare_load() function in ChSystem will take care of this, thank to the new
functions VariablesFbIncrementMq() that have been added to all ChPhysicsItem objects.
The reason: this is required for the new FEM technology in unit_FEM.