Fix sorting of weight vector

Merged Matthew Gignac requested to merge mgignac/athena:hepmcweightsvc-weight-fix into master

HepMCWeightSvc was sorting the weight name map incorrectly. Since HepMCWeightSvc is used to load the weight map when reading GenEvent from the McEventCollection in EVNT files, the weight map that propagated to GenEvent was incorrect.


Tagging @cgutscho, @dhirsch

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