Support for eras: split and combined plots

As discussed in https://gitlab.cern.ch/cp3-cms/bamboo/issues/2#note_2393060 : by default the list of eras is taken from the YAML configuration file, and both per-era and combined plots are produced. This can be customised (only split or combined plots, or restricting to some eras) with the --eras option for bambooRun.

This requires the latest plotIt (with https://github.com/cp3-llbb/plotIt/pull/115 merged), and the luminosity should be specified for each era (so analysis configs will need to be updated when picking up this change).