Default "between" argument for PETSc TS solver.

[eslickj]

I have a very minor suggestion on the default "between" argument. Between defaults to every point in the time continuous set. There is also an "interpolate" argument, which. I think defaults to true. I feel like there is some expectation that if interpolate is True that "between" should default to the first and last points in the time set. If it makes things easier, maybe "interpolate" could default to False and then if "interpolate" is True, between could default to first and last time and every time point otherwise. It wouldn't be a big deal except that if between is every time in the continuous set, the solve can be very slow since you need to write a new NL file and restart the solver each time, and unless you know what's going on the sort-of conflict between "interpolate" and "between" may be confusing.