More UL fixes; add PDF uncertainties; use nTightTags as aux variable in unfolding

See commit list for full details (many small things).

When merged you'll need to update correctionlib: pip install --upgrade git+https://github.com/cms-nanoAOD/correctionlib.git@master. Also, don't forget to keep the ttbb-differential/scale-factors up-to-date (cloned in ttbbRun2Bamboo/../, we should use a submodule for that...).

Also note: I added a --syst-samples argument to switch on the processing of systematic samples (off by default). For a full unfolding setup this then needs to be included.