adding trigger-leg-counting function to TrigGlobEffCorrTool

As discussed for potential use by AnalysisTop, this adds a simple function to allow users to obtain the number of legs for a given trigger from its name, since the TrigGlobalEfficiencyCorrectionTool already has such information internally. Prototype:

virtual CP::CorrectionCode countTriggerLegs(const std::string& trigger, std::size_t& numberOfLegs);

Tested, works. It only supports "common" triggers (those with scale factors!), as defined in data/Triggers.cfg --- although new definitions can be added in that configuration file as needed, of course.

Not thread-safe but it's not the only place in the code... so I'll tackle this when migrating the tool to R22.