Update eres syst
There was an issue in the alpha_Eres_* implementation in nmo_cost_function, the modified probabilities wasn't be used to smear the energy resolution, this MR is to fix that.
Also, I have implemented some improvement in the Eres_* syst calculation, which brings 3-4x speed up when running run_nnxssst_NES_FX_syst.py