mixing with premixed events in input

Antanas, this is how I believe we can foresee the premixed events dataset to be put in input to the cmsDriver sequence, when we need it.
I don't have the setup to test this functionality - could you please look at it, and try import this PR into the dev ? We could look together at how it behaves.
Note that the handling of the dataset input for standard workflows has not been touched.
NOT FOR PRODUCTION.