Quick fix for export to JSON
This is just a quick fix to make the JSON export properly working for LinearPETC.
Further on, the repr() function should not be used to return data structures, and, instead, should only return a string, see https://docs.python.org/3/library/functions.html#repr
Currently, similar logic of returning a data structure can be found in ETCetera/Abstractions/NonlinearETC/TrafficModelNonlinearETC.py as well. I guess that is incorrect and should be later fixed.