Standardize arguments to various types of functions

[jonbinney]

Over time, we've accumulated multiple different functions that do the same thing in different ways. I'd like to push towards having a standard set of arguments for them, and a standard return value. That will let us treat the functions as interchangeable when using them. We don't need to do this for all functions, but I think the ones that would make sense would be:

1. s-parameterization functions - functions that take a path as a list of waypoints in joint space and return a PiecewiseFunction
2. time-parameterrization function - functions that take the result of 1 as well as min/max jerk/acc/vel values, and return a PiecewiseFunction that maps from t to s.

There are probably more I'm missing right now.