Added first informal parser interface, only requiring a method to parse
events and one to parse trackers.
Theoretically we only *require* a method to parse events since, through
their contained activities, they would also come with trackers.
But this would 1) be much more opaque and a lot of work to then extract
the trackers again and 2) leave out trackers which do not yet have any
activities associated with them (i.e. trackers never once accomplished).
We can still turn the informal parser into a formal interface if need
arises: https://realpython.com/python-interface/
