CMSSW_9_3_15

Changes since CMSSW_9_3_14_patch1:

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CMSDIST Changes between Tags REL/CMSSW_9_3_14_patch1/slc6_amd64_gcc630 and REL/CMSSW_9_3_15/slc6_amd64_gcc630:

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1. 4822 from @SiewYan: backporting sherpa2.2.6 and openloops2.0.0 to 93X created: 2019-03-28 22:15:01 merged: 2019-05-07 17:02:18