Delete old post-processor code.

[sniyaz]

Now that we've merged #579 and all upstream PRs, it's time to delete the old PP functionality before we had these nice new methods. Specifically, we're throwing out smoothPath, retimePath, retimePathWithKunz, and the old version of getTrajectoryPostProcessor that was hardcoded to return a ParabolicSmoother.

This will break lots of things upstream, but we'll handle those separately.

---

Before creating a pull request

• Document new methods and classes
• Format code with make format

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change

[sniyaz]

@brianhou @egordon One open question right now for y'all: I went ahead and deleted the methods from ConcreteManipulator as well. Do we want to "pass through" the new PP methods (postProcessPath) in this class as well? Or is a simple deletion fine?

[sniyaz]

@brianhou I just sent out the libherb and herb3_demos PRs. Doesn't look like we ever used those methods in ConcreteManipulator anyway, and everything works.

If this looks fine, can we merge this soon?

[brianhou]

Thanks for making these changes and propagating them downstream! Super excited to delete code 😄

I'll merge once Travis is happy and then release 0.4.0.