Use ROOT histograms for the threshold scan analysis
Description
As the title says and as discussed in private, the threshold scan analysis should take ROOT histograms (and thus ROOT files) as input instead of the current CSV-like file. This is similar to what is being done for the S-curves in !47 (merged).
Getting a reasonable input dataset is strictly dependent on cmsgemos!312 (merged) - without a proper fix, threshold scans cannot be taken at P5 - and partially dependent on cmsgemos!305 (merged) - the MR does work but should be finalized and merged in.