Update PMGTruthWeightTool and add PMGTruthWeightAlg

This is an attempt to add an algorithm processing generator weights using PMGTruthWeightTool and decorating them as systematics.

There have been several changes to the tool done:

• new interface member getSysWeight to get the weight of current systematics
• it now bases on CP::ISystematicsTool
• all names handling is done on initialise
• on beginInputFile we fail if another MC sample is opened
• systematic names are postprocessed and mapped when loading the names, a separate map used is for weight names and for systematics names

I'm open for additional input.

/cc @krumnack @jburr @lcorpe @shuli