Minimalistic tree-based analysis

Add a new analysis that produces trees as opposed to histograms. It implements the baseline event selection from this talk, which is good for both the main analysis and an eμ control region. The list of stored variables is currently minimalistic, and theory uncertainties are not included. Harvesting works with produced files, but the plotting script, of course, doesn't.

This analysis should eventually replace the current histogram-based one. Producing trees is a must for a multivariate analysis. In addition, all systematic variations encoded in event weights can be evaluated at the same time instead of rerunning the whole analysis.

Update: In 0622705e, implemented (optional) storing of four-momenta of jets and leptons, which is useful to construct higher-level observables. With this, the list of stored variables is not that minimalistic anymore.