Importing logsumexp from scipy.special in Multistateanalyzer

[dprada]

There was a logsumexp left to be fixed: in the file multistateanalyzer.
This was probably the source of the errors reported by Travis CI.

[jchodera]

Thanks for the fix, @dprada! We migrated YANK to use the new openmmtools.multistate functionality, but haven't yet removed those files from the YANK repo, so it looks like we need to fix it in http://github.com/choderalab/openmmtools instead

[andrrizzi]

Looks great, thank you! I think we can merge this and then wait for an openmmtools bugfix release maybe (and let tests pass) before merging #1160.