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/cpp/frc2/command/SequentialCommandGroup.cpp

@@ -0,0 +1,73 @@
+/*----------------------------------------------------------------------------*/
+/* 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#include "frc2/command/SequentialCommandGroup.h"
+
+using namespace frc2;
+
+SequentialCommandGroup::SequentialCommandGroup(
+    std::vector<std::unique_ptr<Command>>&& commands) {
+  AddCommands(std::move(commands));
+}
+
+void SequentialCommandGroup::Initialize() {
+  m_currentCommandIndex = 0;
+
+  if (!m_commands.empty()) {
+    m_commands[0]->Initialize();
+  }
+}
+
+void SequentialCommandGroup::Execute() {
+  if (m_commands.empty()) return;
+
+  auto& currentCommand = m_commands[m_currentCommandIndex];
+
+  currentCommand->Execute();
+  if (currentCommand->IsFinished()) {
+    currentCommand->End(false);
+    m_currentCommandIndex++;
+    if (m_currentCommandIndex < m_commands.size()) {
+      m_commands[m_currentCommandIndex]->Initialize();
+    }
+  }
+}
+
+void SequentialCommandGroup::End(bool interrupted) {
+  if (interrupted && !m_commands.empty()) {
+    m_commands[m_currentCommandIndex]->End(interrupted);
+  }
+  m_currentCommandIndex = invalid_index;
+}
+
+bool SequentialCommandGroup::IsFinished() {
+  return m_currentCommandIndex == m_commands.size();
+}
+
+bool SequentialCommandGroup::RunsWhenDisabled() const {
+  return m_runWhenDisabled;
+}
+
+void SequentialCommandGroup::AddCommands(
+    std::vector<std::unique_ptr<Command>>&& commands) {
+  if (!RequireUngrouped(commands)) {
+    return;
+  }
+
+  if (m_currentCommandIndex != invalid_index) {
+    wpi_setWPIErrorWithContext(CommandIllegalUse,
+                               "Commands cannot be added to a CommandGroup "
+                               "while the group is running");
+  }
+
+  for (auto&& command : commands) {
+    command->SetGrouped(true);
+    AddRequirements(command->GetRequirements());
+    m_runWhenDisabled &= command->RunsWhenDisabled();
+    m_commands.emplace_back(std::move(command));
+  }
+}