check_hft_trees: open the file within fillAndCount
check_hft_trees : define Sample class taking care of inputs, filename, treename, etc.
first version of mljj plot (does not include error bands yet)
plotFakeClosure: move some functions to systUtils
define the list of variations
define rootUtils.setWhPlotStyle
check_hft_trees: major reshuffling

- Add all the necessary cmd-line options
- Refactor the functions used to fill the histos and counters so that
  I can re-use them for each sys variation
- refactor Sample class, so that now provides the correct hft
  filenames and treenames (change their value only when dealing with a
  syst that is relevant for this sample; use nominal otherwise)