This causes setVoltage to be called on the lower level motor contollers,
which is benefical in cases when they are smart motor controllers.
Previously, the default implementation (using the bus voltage and
calling set()) was used in this case.
This does slightly pessimize the case when the lower level motor
controllers use the default setVoltage implementation, but given the
prevalence of smart motor controllers, this seems like an overall win.
The non-NT portion has been moved to wpiutil.
The NT portion has been moved to ntcore (as NTSendable).
SendableBuilder similarly split and moved.
SendableRegistry moved to wpiutil.
In C++, SendableHelper also moved to wpiutil.
This enables use of Sendable from wpimath and also enables
moving several classes from wpilib to wpimath.
Also deprecate SpeedController in favor of motorcontrol.MotorController and
SpeedControllerGroup in favor of motorcontrol.MotorControllerGroup.
The MotorController interface is derived from the SpeedController interface
so that code such as SpeedController x = new VictorSP(1) continues to
compile (just with a warning).
SpeedControllerGroup and MotorControllerGroup are independent classes;
both implement the MotorController interface.
