Add bamboo.analysisutils.makeMultiPrimaryDatasetTriggerSelection method

Fixes https://gitlab.cern.ch/cp3-cms/bamboo/issues/25.

Testing done: the results from

makeMultiPrimaryDatasetTriggerSelection("MuonEG_2016F", datsetAndTrigSel) ## also DoubleMuon_2016F, DoubleEG_2016F

looks reasonable (with the primary dataset / trigger combinations from the example in the docstring)