Fix poisson significance plotting

Release notes

Recommendation for significance calculations is provided on slide 9 https://indico.cern.ch/event/768968/contributions/3195413/attachments/ 1743878/2822499/statsCommitteeAtlasWeeklyOct2018.pdf This MR adds significance calculation for this poisson significance with account of background error.

Details

Histogram subplot and cutflows are currently covered.