Fix incorrect AWH free-energy output with multiple walkers

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