For signals, we still use a json file to map a signal to a specific tree that we use to count the number of saved events.
Now, the keys in the json file don't need to be an exact match for the full file name; rather, the key should be a substring of the file name.
When we count events for a signal ROOT file, the function GetMatchingKey() is used to find the matching key (substring) for that ROOT file (specific to the signal sample).
This makes the keys much more flexible and general.
Event counting: improvement for signals