Add script for adding up the xsecs in runpoint_*/result.log

Added a script that sum up the xsec values & errors in result.log's. Used the same strategy in GenXsecAnalyzer: https://github.com/cms-sw/cmssw/blob/master/GeneratorInterface/Core/plugins/GenXSecAnalyzer.cc#L411-L474 which calculates matching effi. & errors and estimate final matched xsec & error.

Currently only assumed using in jet-binned samples, so you should provide two inputs: e.g.) ./genval-xsec --datasetname DY012jNLO_MG273 --njets 2