Add copyright header

dune/common/copyableoptional.hh

 // -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 // vi: set et ts=4 sw=2 sts=2:
+// SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 #ifndef DUNE_COMMON_COPYABLE_OPTIONAL_HH
 #define DUNE_COMMON_COPYABLE_OPTIONAL_HH
 

dune/common/test/copyableoptionaltest.cc

 // -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 // vi: set et ts=4 sw=2 sts=2:
+// SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 
 #ifdef HAVE_CONFIG_H
 #include "config.h"