Feature/improve nuts diagnostic

Introduced a new NUTS statistic that helps to identify over-aggressive adaptation (and tweaked the acceptance stat to use all samples in the NUTS tree instead of just those in the last subtree).  Documentation updated and test-unit passes.

I know it may not be possible to modify the Jenkins queue all that easily, but the changes are relevant for some of the problems users are seeing and might be worthy of elevation to higher priority if deemed appropriate.