Add Poisson significance with background uncertainty to gridscanner

Release notes

Add Poisson significance with background uncertainty to gridscanner, with option "poissonwerr"

Details

Hi @bejaeger , hope you are the right person to tag - git blame tells me that you have worked on the grid scanner code.

For the heavy majorana neutrino search, we are interested in using the gridscanner (which btw is a really nice tool!) for our signal region optimisation. The Poisson significance which accounts for the uncertainty on the background uncertainty is not implemented with this tool, and this is the figure of merit we would like to use. Particularly as we will be looking into tails of signal distributions where the predicted background could have limited statistics. This MR adds this functionality, hope it's OK.

Thanks!