Major refactor

[Radonirinaunimi]

This PR implement a few major refactoring:

• It trades the bin-wise computations that introduced unnecessary complications (such as having to deal with $Q^2$-nodes) in favor of point-wise calculations
• Introduces the possibility to explicitly account for the renormalization scale $\mu_R^2$ dependence in the grids

For the record, we may want to revisit the bin-wise implementations in the future, and the stable reference for this is 1de0a2e.