Refactor reroute_ts() to be less error-prone.

[frreiss]

Every time that I've used the reroute_ts() function in reroute.py, I've ended up putting the arguments in the wrong order. Mixing up the ts0 and ts1 args to this function doesn't generally lead to errors, but to an incorrect modification of the graph that in turn leads to strange problems downstream in the user's code. We should update the documentation, the API, and the error-handling to make this kind of coding error less likely.