Removing the source+bkg merging step

[GallegoSav]

It is an extra step which can take time when you have an heavy file and not necessary at all.
And also it ducplicates the number of needed files for tutorial. Instead people could just download bkg and signal file and define the data as signal.binned_data.project('Em', 'Phi', 'PsiChi')+bkg.binned_data.project('Em', 'Phi', 'PsiChi').

Like for the crab :

cosi = COSILike("cosi",                                                        # COSI 3ML plugin
                 dr = dr,                                                      # detector response
                 data = crab.binned_data.project('Em', 'Phi', 'PsiChi')+bkg.binned_data.project('Em', 'Phi', 'PsiChi'),   # data (source+background)
                 bkg = bkg.binned_data.project('Em', 'Phi', 'PsiChi'),         # background model 
                 sc_orientation = sc_orientation,                              # spacecraft orientation
                 nuisance_param = bkg_par,                                     # background parameter
                 earth_occ = True)                                             # Option to account for Earth occultation

This would require updating all the tutorials and removing this extra step and all the signal+bkg combined file though