Fix bug in the definition of the 1D luminosity.
This PR addresses #1442 (closed) (see explanation there). A quick computation of the gg 1D luminosity plot (see attachment) reveals that it is the same as the one computed with APFELweb.
validphys
APFELweb
Additional checks must be carried out to verify that luminosities are indeed defined as in Eqs. (9.1)-(9.2)-(9.3) of the NNPDF4.0 paper.