Implement toy LHAPDF PDF set

Created by: scarrazza

Issue by Zaharid Friday Dec 01, 2017 at 15:56 GMT Originally opened as https://github.com/NNPDF/validphys2/issues/68

---

It would be nice to have a toy PDF where things like the and the arclength and the momentum fractions are known, so that we can easily have tests. Ideally it would be in LHAPDF format so we get to exercise the same code paths that we use now.