Code review comment for lp://staging/~madnklo/mg5amcnlo/MadNkLO_ISR

New contributions dubbed 'beam-factorization' contributions (BF) are introduced, together with their corresponding PDF counterterms.
These are generated automatically by inspecting existing contributions at lower orders, so that this part of the code is N^kLO ready already in principle.

The workflow of the __call__ and sigma functions of the ME7Integrands has been significantly modified/improved in order to have a more streamlined processing of the flavour mapping and back-propagation in ISR counterterms.
This improvement is realised with the new concept that the 'sigma' function no longer returns a single weight but instead a collection of 'ME7Event', each containing a list o weights for all the flavour structures it can adopt.

The application of the cuts and observables is then much more elegant as it is directly applied to the overall list of events generated for one specific set of input variables in the unit hypercube.

Given this large structural change, it is desirable to merge this branch sooner than later in the MadNkLO trunk. It is however not complete as it must still be tested beyond 2>1, at NNLO too, and the finite part of the integrated soft and soft-collinear counterterms is still missing.

This is however mostly irrelevant w.r.t this merge because the structural changes that were necessary for ISR are now established.