docs(license): add SPDX identifier to source files

src/Init/GridInit.cpp

@@ -1,3 +1,9 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * GridInit.cpp - Implementation of grid initialization
+ * Copyright (C) 2024-2025 Max Luebke (University of Potsdam)
+ */
+
 #include "InitialList.hpp"
 
 #include "PhreeqcMatrix.hpp"

src/Transport/DiffusionModule.cpp

@@ -1,25 +1,8 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
 /*
-** Copyright (C) 2018-2021 Alexander Lindemann, Max Luebke (University of
+ * DiffusionModule.cpp - Coupling module betweeen POET and tug
-** Potsdam)
+ * Copyright (C) 2018-2025 Max Luebke (University of Potsdam)
-**
+ */
-** Copyright (C) 2018-2022 Marco De Lucia, Max Luebke (GFZ Potsdam)
-**
-** Copyright (C) 2023-2024 Marco De Lucia (GFZ Potsdam), Max Luebke (University
-** of Potsdam)
-**
-** POET is free software; you can redistribute it and/or modify it under the
-** terms of the GNU General Public License as published by the Free Software
-** Foundation; either version 2 of the License, or (at your option) any later
-** version.
-**
-** POET is distributed in the hope that it will be useful, but WITHOUT ANY
-** WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
-** A PARTICULAR PURPOSE. See the GNU General Public License for more details.
-**
-** You should have received a copy of the GNU General Public License along with
-** this program; if not, write to the Free Software Foundation, Inc., 51
-** Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
-*/
 
 #include "DiffusionModule.hpp"