various time integration methods for the finite elements\n
\section description
gear - atomic velocity update with 2nd order Verlet, nodal temperature update with 3rd or 4th order Gear, thermostats based on controlling power \n
fractional_step - atomic velocity update with 2nd order Verlet, mixed nodal temperature update, 3/4 Gear for continuum and 2 Verlet for atomic contributions, thermostats based on controlling discrete energy changes\n