Support for background processes in unfolding

The contribution of background processes is usually subtracted from the reco-level histograms prior to unfolding. This is apparently done by repeatedly calling TUnfoldDensity::SubtractBackground with the background histograms and normalization + errors (see also #87).

Systematic (shape) uncertainties in the backgrounds don't seem to be supported natively by TUnfold so they would need manual handling. But I see there is a loop over systematics in unfold, so maybe TUnfold's native support for uncertainties is not used after all?

Development is needed at least for the following:

• Store background contributions in the relevant histograms in getUnfHist
• Subtract backgrounds from the observed distributions in unfold (or even in a separate executable prior to unfolding? this could be elegant if feasible... depends how uncertainties are handled)