Update cosine.ipynb, MC/COSINE_SET2_BackgroundModel_C2.csv,...

Update cosine.ipynb, MC/COSINE_SET2_BackgroundModel_C2.csv, MC/COSINE_SET2_BackgroundModel_C3.csv, MC/COSINE_SET2_BackgroundModel_C4.csv, MC/COSINE_SET2_BackgroundModel_C6.csv, MC/COSINE_SET2_BackgroundModel_C7.csv, data/COSINE_SET2_Data_C2.csv, data/COSINE_SET2_Data_C3.csv, data/COSINE_SET2_Data_C4.csv, data/COSINE_SET2_Data_C6.csv, data/COSINE_SET2_Data_C7.csv, figs/background_modeling.png, figs/exclusion_limits.png, figs/fitting_result_spectra.png, fit/fit_Result.csv, limit/COSINE_SET1_Limit_Isospin-Conserving_SI_WIMP.csv, limit/COSINE_SET1_Limit_Isospin-Violating_SI_WIMP_fnfp-071.csv, limit/COSINE_SET1_Limit_Isospin-Violating_SI_WIMP_fnfp-076.csv, limit/COSINE_SET2_Limit_Isospin-Conserving_SI_WIMP.csv, limit/COSINE_SET2_Limit_Isospin-Violating_SI_WIMP_fnfp-071.csv, limit/COSINE_SET2_Limit_Isospin-Violating_SI_WIMP_fnfp-076.csv, limit/COSINE_SET2_Sensitivity_Isospin-Conserving_SI_WIMP.csv, limit/COSINE_SET2_Sensitivity_Isospin-Violating_SI_WIMP_fnfp-071.csv, limit/COSINE_SET2_Sensitivity_Isospin-Violating_SI_WIMP_fnfp-076.csv