sigproflik -> z_calculator, which unifies the sample
xsecs/nevents/lumis, input files, weights, and getting event counts
per bin, in addition to doing the PL calculation;

signal_xsec default -> 20 fb;
bigw and syst_frac now options;
nsig/bkg_nm1/total managed by z_calculator class, per sample;
no longer use weighted files, but instead add ourselves in
z_calculator (and remove weight_files function).
samples have colors; samples_by_name map.
total_hist returns a stack for showing the separate background contributions.