plotter: for now only look at SS region
todo reminder (should modify met)
drop inputdir option

Details:
use the same approach as in plotTightProbability.py. Inputs are
required arguments, which can be either directories or files.
drop useless assert

Details:
the files are already filtered by glob (only existing ones), so this
assertion is now useless.  In fact, it was useless already before,
because I was not checking the return value of TFile.Open, just the
length of the list.
drop obsolete tag option
replace guessSampleFromFilename with guessGroupFromFilename

Details:
with the new datasets implementation, guessSampleFromFilename is now
obsolete. Replace it with guessGroupFromFilename, that works with
filenames for both merged and unmerged samples.
switch guessSampleFromFilename -> guessGroupFromFilename