Adding 421305 Wmunu Sherpa 2.2.8 validation sample

Matthew Gignacrequested to merge
dsid_mgignac_421305 into master

Weak boson group would like to provide validation samples for Wmunu in Sherpa 2.2.8;

JIRA for the submission : https://its.cern.ch/jira/browse/AGENE-1772

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