Use "signal tag" to distinguish alternative signal samples

Note: based on top of !39 (merged), which is to be merged first

This is the first (and easiest ;) ) step in solving https://gitlab.cern.ch/kcormier/uzh-ttbb-unfolding/-/issues/4

Assign a different "tag" to the various signal samples. At the moment we only have two (powheg_4FS, powheg_5FS), but there will be more. The "main" signal sample can be chosen using a command-line argument to bambooRun, but the only effect of that in this code is to choose which sample is used for the ttB contribution in the plots produced by plotIt.

Note: feedback welcome: there may be other ways to handle this, as there might be consequences downstream in the unfolding code I haven't thought of...