Ignition simulation

1. Add a new opption in control file.
    t_mode = 3; ignition mode
   For ignition mode, NVT simulation is carried out when a boost applied.
   When boost is turned off, the simulation is changed to NVE simulation.
   This is achieved by adjust the control->T
   When boost is turned off, control->T is updated every step to mimic a
   NVE simulation. If T < ignition T, the T is set to ignition T.