change of timestep in cp.x

Implemented change of timestep during a verlet run using autopilot module.

Added option electron_velocities='change_step' to do exactly the same thing that ion_velocities='change_step' does, but for electronic d.o.f.:

lambdam = lambda - (lambda-lambdam)*delt/dt_old
cm_bgrp = c0_bgrp - (c0_bgrp-cm_bgrp)*delt/dt_old

In this way the change of timestep is smoother.