buildWeightedMatrix.py: add more srnames mapping
utils: fix unit test

Details:
I have now made up my mind, and decided that leading and trailing '_'
are separators, thus not part of the tag. Fixing the unit test
accordingly.
utils: docstring and move functions
bugfix: prevents building output filename with __
cat_ROOT_file: initial import
buildWeightedMatrix: bugfix bk compat *_real_dn -> *real_down
FakePred: use new matrix Oct_30
MatrixPrediction: add new control regions and clarify

Details:
- filling histos for all the new control regions
- clarify that for each control region we are computing the fake
  weight from a different fake selection region, that is with the
  composition derived in a different fake selection region
- still need to make this more explicit by caching all of the common
  arguments (to getFakeWeight and to fillHistos)
add buildWeightedMatrix.py command
json in/out
plot fractions