Feature to determine expected signal strength fit result from toys

This adds a ``--max-likelihood-toys`` option to limit.py which runs toys and then max likelihood fits the toys, according to the prescription at [1].  The toys mu results are then produced in a TTree in the .root output.

The option respects the ``-s`` seed and ``--toys`` options.

[1] https://twiki.cern.ch/twiki/bin/viewauth/CMS/SWGuideHiggsAnalysisCombinedLimit#Toy_by_toy_diagnostics