save all MCs when asking for root format

CMGRDF/plots.py has some interesting notions of signal and background that affect whether or not a given sample are saved, etc. we should discuss more thoroughly what do we want this code to do, how the sample flags should interact with each other, etc.

this PR just changes the code such that all MC samples (as defined by processes in samples.py) are saved into the file when root plot format is requested, regardless of its sig/bkg status.