add run script for amcatnlo tarballs
remove no-long-working workaround for amcatnlo event weights
loosen lhe header compatibility check given that random seeds and number of events may change from job to job for madevent and madspin
Merge pull request #2986 from bendavid/amcatnloprod53x

Changes to allow aMC@NLO production with ExternalLHEProducer (53x)