Add new C++ Command framework (#1785)

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.

wpilibc/src/main/native/include/frc2/command/NotifierCommand.h

@@ -0,0 +1,51 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <frc/Notifier.h>
+
+#include "CommandBase.h"
+#include "CommandHelper.h"
+
+namespace frc2 {
+/**
+ * A command that starts a notifier to run the given runnable periodically in a
+ * separate thread. Has no end condition as-is; either subclass it or use {@link
+ * Command#withTimeout(double)} or
+ * {@link Command#interruptOn(BooleanSupplier)} to give it one.
+ *
+ * <p>WARNING: Do not use this class unless you are confident in your ability to
+ * make the executed code thread-safe.  If you do not know what "thread-safe"
+ * means, that is a good sign that you should not use this class.
+ */
+class NotifierCommand : public CommandHelper<CommandBase, NotifierCommand> {
+ public:
+  /**
+   * Creates a new NotifierCommand.
+   *
+   * @param toRun        the runnable for the notifier to run
+   * @param period       the period at which the notifier should run, in seconds
+   * @param requirements the subsystems required by this command
+   */
+  NotifierCommand(std::function<void()> toRun, double period,
+                  std::initializer_list<Subsystem*> requirements);
+
+  NotifierCommand(NotifierCommand&& other);
+
+  NotifierCommand(const NotifierCommand& other);
+
+  void Initialize() override;
+
+  void End(bool interrupted) override;
+
+ private:
+  std::function<void()> m_toRun;
+  frc::Notifier m_notifier;
+  double m_period;
+};
+}  // namespace frc2