8FUNCTIONS: derms - CYANA function

Synopsis: derms = <real>

Real function that returns the RMS total energy change per timestep,
averaged over all timesteps of the most recently executed 'md'
command. The total energy is given by the sum of the potential energy
(target function) and the kinetic energy. The change of the total
energy from one timestep to the next is calculated before velocity
scaling is applied for controling the temperature.
