[WIP] Operational space ex

The start of an example of operational space control in Simbody. @hsu @scpeters @tkuchida @sohapouya could chime in too. I've added those 4 people and @sherm1 as collaborators on my fork; feel free to directly add commits to this branch. 

Right now, there's a reaching task with gravity compensation in its nullspace. I have coded it up inefficiently and in one method. However, I have spent time trying to do the same calculations efficiently; look at all the commented-out code.

Possible things to do:

- [x] Add methods/operators to Simbody to calculate things like the task space mass matrix, or the dynamically consistent jacobian inverse.
- ~~[ ] Add a Force::OperationalSpace to Simbody that has an interface to build up an op-space controller for those who don't want to have to type out the calculations themselves.~~
- [x] Add documentation in the example, referring people to resources about operational space control.
- ~~[ ] Use an internal model to compute the controls.~~
- ~~[ ] Add noise to model sensing of the state of the "actual" model.~~
- ~~[ ] Model the actuation: model torque sensing and control.~~
- [x] Display goemetry at the target point.
- [x] Mouse or keyboard input to move around the target point.
- [x] use Simbody.h instead of SimTKsimbody.h
- [x] InertialForces should depend on Velocity stage.
- [ ] Optimize.
- [ ] Write test cases (using an RRR robot?).
- [ ] Why does the simulation freeze when the target goes out of reach of the humanoid?

I'm hoping this PR can be a place for discussing exactly what we want this example to be, what new methods we want in Simbody, and what we are doing for the September deadline.

This code is somewhat derived from stuff Gerald Brantner did for ME 485 last Spring.