Rewrite systematics handling in PMGTruthWeightTool
Rewrite the systematics handling in the PMGTruthWeightTool as it was found to be implemented in a very inefficient manner. This involves an interface change for the tool, which now implements a IReentrantSystematicsTool
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See JIRA ticket: https://its.cern.ch/jira/browse/ATLASG-2357