b tag: nicolo's model, fix of parametrized b tagger

[cbernet]

This pull request is mainly about the modelling of b tagging. It includes Nicolò's model and a fix of the parametrized b tagging.

Nicolò's modelling is discussed here: https://github.com/HEP-FCC/heppy/issues/48 There is still work to be done on the tuning and the software integration side, but it is worth starting to use it.

The parametrized b tagging involves a matching of reconstructed jets to the b quarks, to decide whether a jet should be considered as a bjet or not. Previously, this matching was done by delta R in the eta, phi space, and it was fairly inefficient, leading to an artificial degradation of the b tagging efficiency.

Now, it's done in the following way:

• the gen particles corresponding to the reconstructed jet particles are found using @alicerobson history tools
• if a gen particle comes from the decay of a B hadron (again @alicerobson history tool), the corresponding reconstructed particle is flagged as coming from B
• the jet energy fraction carried by reconstructed particles coming from b is computed
• if the fraction is > 0.05, the jet is flagged as a b jet, and we can then apply the chosen detector performance.

The matching efficiency is now ~99% (depends on the jet energy), for a mismatching rate (probability to flag a light jet as coming from a b) of about 10^-3. Therefore, one can consider that the matching does not bias the b tagging efficiency anymore.

The other features affect analyses I'm the only one to perform at the moment and are not worth mentioning.

@kbehr @JanikvA @jndrf

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