TrackRungeKuttaExtrapolator: Implement both `toStream` and `toString` instead of just `toStream`
toStream
must produce a valid python identifier, which in this case
implies that it must add quotes so its output is a (python) string.
toString
on the other hand must produce something that fromString
can
parse, and thus it should not include the quotes.
Note: this depends on gaudi/Gaudi!372 (merged) to get the ADL lookup of toString
correct...