Extend n3fit and vp to deal with two different initial state distributions

Created by: Radonirinaunimi

The following PR modifies both n3fit and vp to deal with the case in which (SI)DIS/DY processes involve convolutions between two different distributions (PDF, pPDF, FF, etc.). A simple example of such a scenario is the case in which one of the protons is polarized and the other not:

\Delta \mathcal{O} = \sum \Delta f \otimes f \otimes \Delta \Sigma

This PR introduces a feature available since pineappl <v0.8.0 that specifies the type of convolution required by an initial state with the convolution_type_ key (whose value can be UnpolPDF, PolPDF, FF).

The main modifications affect:

• the new convolution that includes the
  \Delta V
  and
  \Delta V_3
  distributions
• the convolution during the fits in which one of the distributions is computed from a boundary condition
• the computation of the theory predictions in vp
  • \chi^2
    -computations
  • data vs theory comparisons

Minimal non-congruent modifications:

• deprecate the is_polarized logical
• modify the polarized Positivity commondata to impose the BCs on the individual pPDF flavors

To do at the very last stage:

• re-generate the regression tests

Rely on https://github.com/NNPDF/pineappl/pull/289, https://github.com/NNPDF/pineko/pull/181.