[wpilib] Fix bugs in Hatchbot examples (#2893)

This fixes an issue with some commands not correctly requiring their
subsytems. Furthermore, an execute() method was added to the
DriveDistance command to continuously update the voltage command.

wpilibcExamples/src/main/cpp/examples/HatchbotTraditional/include/commands/DriveDistance.h

@@ -1,5 +1,5 @@
 /*----------------------------------------------------------------------------*/
-/* Copyright (c) 2019 FIRST. All Rights Reserved.                             */
+/* Copyright (c) 2019-2020 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.                                                               */
@@ -26,6 +26,8 @@ class DriveDistance
 
   void Initialize() override;
 
+  void Execute() override;
+
   void End(bool interrupted) override;
 
   bool IsFinished() override;