Enable outputting average pull forces (and positions)
When pulling under nonequilibrium conditions it is often not enough to write e.g. pull forces every 100 steps (or even less frequently). Usually the forces (and/or coordinates) are written every step to properly catch all events etc. However, this makes the output files very large and makes analysis slow. Enabling writing the average force (or coordinate) over a period instead of the force (or coordinate) at the specific step of the output circumvents this problem.