Automated Verlet rlist for NVE simulations

NVE simulations with the Verlet scheme will now automatically have
the list buffer set using verlet-buffer-tolerance and the initial
temperature, unless this is zero in which case a 10% buffer is used.
A warning is added for potentially large drift over the simulation.
This change makes empty mdp files valid again.

Change-Id: Iae2d3d8ba212dd9e57783d9fb3d84f7cfc1efdd0