Introduce a new executable supporting user-defined event skims

Motivation

We want to combine two dataset with no overlapping phase space. So a tool is need to allows analysts to quickly test different selection criteria without modifying and recompiling the code.

Summary

This merge request implements a generic event-level selection mechanism based on user-defined cuts using TTreeFormula. The input cut expression can be written in the same format as used in TTree::Scan, e.g., recJets[0].CorrPt() < 80 && @recJets.size() > 2.

Improve exception handling. Some foreseeable errors may occur

• segmentation violation: recJets[4]<40 while there are only 3 jets in the events
• Unknown branch or syntax error

This version is just a quick prototype and currently only supports simple expressions like recJets[0].CorrPt() < 80. Feel free to share any suggestions or feature requests.