Implementation of muon id and iso scale factors

Created by: mmarchegiani

The computation of the weights associated with the muon identification and isolation cut are included in the framework, and applied to Monte Carlo. The scale factors are computed for each muon and two per-event scale factors (sf_mu_id and sf_mu_iso) are derived by multiplying the scale factors of all the muons in the event. As in the electron case, two dedicated weights are included in the Weights() object and therefore the muon weights are properly taken into account when histograms are filled. Together with the "nominal" scale factors, also the "up" and "down" variations are stored, allowing future studies on the variation of the corresponding systematic uncertainties.