Separate photon's elastic, inelastic and msbar components

Created by: niclaurenti

This PR aim is the following: we want to provide PDF sets in which, instead of the total photon, there is only its elastic or inelastic component. However, in the sum rules and in the FKtables contraction we still want to have the total component. So, the elastic/inelastic part must be called only at the very last moment, when we register the photon in the lhapdf grid.

At the moment I still don't know how we should treat the additional errors, since they are supposed to be the errors of the total pdf and not of a component of it.