Modify MARLEY gen to account for nadir-energy dependence

[x] Produce TH2D with event probability (Guilherme)
[x] Source a 2D histogram with survival probability as a function of nadir and sample direction and energy of the neutrino
[x] Store in the output the true value of the nadir angle and of the neutrino energy (this last should already be ok)
[x] Decouple angular and energy sampling
[x] Introduce angular direction sampling
[ ] Simulate a flat energy distribution with the solar angular distribution (keep in mind that reweighting may be inefficient in the tail)
[ ] Construct response model (E_ν vs E_vis)
[ ] Write utility to create TH2 from the full one selectin only certain regions in the nadir-energy space.

SoLAr-Neutrinos / SOLAr-sim