Fixing parameters in pyhf and root fits

[danikam]

While doing the validation for the displaced slepton analysis, it was found that it would be helpful to fix the mu_Sig parameter to 0 to do a bkg-only fit when comparing the fitted nuisance params in compare_fitted_nuisance.py . To do so, the following mods were needed:

line 58: bestfit = pyhf.infer.mle.fit(data, model) --> bestfit = pyhf.infer.mle.fixed_poi_fit(0.0, data, model)

After line 25: workspace.var('mu_Sig').setVal(0.0)