Split the two command implementations into separate libraries (#2012)

This will allow us at the user code side to determine to include old commands, new commands or both.

wpilibNewCommands/src/main/native/include/frc2/command/CommandGroupBase.h

@@ -0,0 +1,62 @@
+/*----------------------------------------------------------------------------*/
+/* 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/ErrorBase.h>
+
+#include <memory>
+#include <set>
+#include <vector>
+
+#include "CommandBase.h"
+
+namespace frc2 {
+
+/**
+ * A base for CommandGroups.  Statically tracks commands that have been
+ * allocated to groups to ensure those commands are not also used independently,
+ * which can result in inconsistent command state and unpredictable execution.
+ */
+class CommandGroupBase : public CommandBase {
+ public:
+  /**
+   * Requires that the specified command not have been already allocated to a
+   * CommandGroup. Reports an error if the command is already grouped.
+   *
+   * @param commands The command to check
+   * @return True if all the command is ungrouped.
+   */
+  static bool RequireUngrouped(Command& command);
+
+  /**
+   * Requires that the specified commands not have been already allocated to a
+   * CommandGroup. Reports an error if any of the commands are already grouped.
+   *
+   * @param commands The commands to check
+   * @return True if all the commands are ungrouped.
+   */
+  static bool RequireUngrouped(wpi::ArrayRef<std::unique_ptr<Command>>);
+
+  /**
+   * Requires that the specified commands not have been already allocated to a
+   * CommandGroup. Reports an error if any of the commands are already grouped.
+   *
+   * @param commands The commands to check
+   * @return True if all the commands are ungrouped.
+   */
+  static bool RequireUngrouped(std::initializer_list<Command*>);
+
+  /**
+   * Adds the given commands to the command group.
+   *
+   * @param commands The commands to add.
+   */
+  virtual void AddCommands(
+      std::vector<std::unique_ptr<Command>>&& commands) = 0;
+};
+}  // namespace frc2