Allow different matching scale for the photon

Created by: niclaurenti

Opening this PR to allow different matching scale for the photon. With matching scale here I mean the scale at which the photon is generated (that now is hardcoded to 100 GeV), and not the scale at which we change nf in the VFNS (I know, it is a bit confusing). The matching scale will be read from the eko_photon.tar so it will not be neither in the runcard, nor in the theory card. It means that there will be some copies of theory 523 that will change only for eko_photon.tar.

Honestly, I don't like very much the idea that in order to find out what the matching scale is for a given theory, one has to open eko_photon.tar and read eko_photon.mu20 so if you have better ideas please let me know

edit: in this PR i will also upgrade the photon branch to eko 0.13