Add a voltage-compensated setVoltage method to SpeedController, which is sorely needed to help teams use feedforward-based controls effectively. Also uses correct units on the cpp side.
Also update relevant examples.
This is the C++ version of #1682.
The old command framework is still available, but will be deprecated.
Due to name conflicts, the new framework is in the frc2 namespace.
Eventually (after the old command framework is removed in a future year)
it will be moved into the main frc namespace.
