infiles: list of input dataCollection files to be opened
nprocs: number of processors to use
add_ys: list of additional Y targets to be added at generator time, must be the same length of the input collections. The list content must be iterable, each iteration produces a new target, only scalar type supprted for now
flags: like add_ys, same rules apply. The flags gets multiplied by the event weight is case weights are used. The lenght of the flags must be the same as the TOTAL number of Y targets. Flags are returned instead of the event weights