Add setVoltage method to SpeedController (#1997)

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.

wpilibc/src/main/native/include/frc/SpeedController.h

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2008-2018 FIRST. All Rights Reserved.                        */
+/* Copyright (c) 2008-2019 FIRST. All Rights Reserved.                        */
 /* Open Source Software - may be modified and shared by FRC teams. The code   */
 /* must be accompanied by the FIRST BSD license file in the root directory of */
 /* the project.                                                               */
@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include <units/units.h>
+
 #include "frc/PIDOutput.h"
 
 namespace frc {
@@ -25,6 +27,20 @@ class SpeedController : public PIDOutput {
    */
   virtual void Set(double speed) = 0;
 
+  /**
+   * Sets the voltage output of the SpeedController.  Compensates for
+   * the current bus voltage to ensure that the desired voltage is output even
+   * if the battery voltage is below 12V - highly useful when the voltage
+   * outputs are "meaningful" (e.g. they come from a feedforward calculation).
+   *
+   * <p>NOTE: This function *must* be called regularly in order for voltage
+   * compensation to work properly - unlike the ordinary set function, it is not
+   * "set it and forget it."
+   *
+   * @param output The voltage to output.
+   */
+  virtual void SetVoltage(units::volt_t output);
+
   /**
    * Common interface for getting the current set speed of a speed controller.
    *