compute_fake_compositions.py: allow to specify selection region

ssinc1j was hardcoded
drop old fraction plotting with matplotlib
build_fake_matrices.py: add option region (was ssinc1j hardcoded)
pick&merge: use new Jul_26 files
script with all the steps to build the fake matrix from the nutples

Initial working version, tested for ssinc1j and emu.
Now going to test it also for the regions required by the ss ewk team.