add alsoplaceholders option

Details:
this option is needed to correctly merge the diboson group when
computing the fake (using WZ Powheg instead of Sherpa).  Also add a
warning/suggestion about the (potentially) wrong tag identified when
guessing from the latest root files.
drop debug printout
FakeClosurePlot : only loop on the plotRegions we need
FakeClosurePlot: when missing input histos, warn, skip, and go on
FakePlotting::setOuputDir: add final slash if needed
FakePlotting::init: prepend _ to tag
FinalNewFake::initIoFiles: prepend _ to tag
initial summary