Materials for the PineAPPL v1 paper

[Radonirinaunimi]

This PR adds minor modifications and new changes needed for the PineAPPL v1 paper. In particular, it adds:

• two more scale variation prescription in presence of fragmentation scale:
  • 7-point prescription: similar to the 7-point prescription in the case of two scale with the factorization and fragmentation scales correlated $\left( \mu_F = \mu_A \right)$. This is for example used in https://arxiv.org/pdf/2405.17540 and https://arxiv.org/pdf/2404.08597
  • 13-point prescription: all the three scales are varied independently, spanning all the combinations except the extreme diagonal ones (where the ratio between scales $\mu_i / \mu_j = 4$ or $1/4$). This would be the analogous of the standard 7-point prescription in the case of three scales, and this is what we'd use for example to estimate MHOUs in PDF fits.
• two examples (in C++ and Python) that illustrate the filling of LO predictions for a single-inclusive pion production in proton-proton collision. Here, we should check that the Monte Carlo simulations (esp. the kinematic parts) make sense.