Verify BAR is working with constant energy states

[mrshirts]

if all the w_F = k (the same value), and w_R's are all -k, then everything is consistent - the free energy difference is k, there is no entropy difference, it's just an energy difference. However, some versions of BAR seems to be failing with an error. This needs to be investigated a bit more (by @mrshirts). It's a pretty rare case, but should be caught.