gmx mdrun -append now requires that a checkpoint is found#
gmx mdrun -append would start from the .tpr
configuration (and thus not append) when the checkpoint file was missing.
The Verlet buffer now correctly handles perturbed constraints#
With free-energy calculations with perturbed constraints, the Verlet buffer could be underestimated when constraint lengths were perturbed. As usually only very few constraints are perturbed, the effect is very small and much smaller than the overestimate of the buffer due to approximations, so the results of most runs with perturbed constraints will not have been affected.