Fix incorrect AWH free-energy output with multiple walkers

The free-energy was averaged exponentially over walkers using the wrong sign for the exponent. The meant that the free-energy output was off for large update sizes. This error was quadratic in the update interval and the number of walkers. This likely has no measurable effects with the default AWH free-energy update interval. But the effect is noticeable with the not uncommon update interval of 1000 steps.

Fixes: #3828 (closed)