Update actuators to new API

As a result of discussions on #106, we decided that version 1.1 was a good milestone to update the API for our actuators, giving classes more accurate names, and splitting up physical components, actuators, and low level controllers. The resulting new API was specified here: https://docs.google.com/document/d/1rP1fBWqkQZB7QJGEkUejbUr1A48Rzr25VPNwqMrGfEs/edit?usp=sharing
and are implemented in this pull request.

To summarize, here are the main changes from master:

* Muscle2P is now tgBulletSpringCable
* MuscleNP has moved to core, and is now tgBulletContactSpringCable
* MuscleAnchor is now tgBulletSpringCableAnchor
* tgBaseString is now tgSpringCableActuator
* tgLinearString(Info) is now tgBasicActuator(Info)
* ImpedanceControl has been moved to controllers, and is now tgImpedanceController

Additionally, to address #107, the erroneous acceleration limit has been removed from tgBasicActuator. The tgSpringCableActuator::Config structs within NTRT have already been updated (sorry to do that twice in a few weeks!), but you can expect behavior changes as a result of this. If you want to stick with tgBasicActuator and get the old behavior, I'd recommend decreasing the velocity constraint. I've tagged classes that were using the acceleration constraint with a todo.

Otherwise, if you want properly enforced actuator constraints, take a look at the new tgKinematicActuator. It implements a simple motor model with a linear torque speed curve. It requires a more sophisticated low level controller, so there is now a PID controller class in the controllers directory. tgImpedanceController interfaces both with these low level controllers and directly with tgBasicActuator via the new tgTensionController (which replaces tgLinearString::tensionMinLengthController). There is also a new base class tgControllable to provide a base class to pass to these low level controllers.

The existing examples have been set up to use this new API, and we'll get some new examples up in the coming weeks.

This pull request includes all of the commits from #106, so that one can be closed if this one is merged in.