OTH: added a header to be included in all files.

Contains exclusion of warranty, reference to licence, and SAMPLE
citation

header.txt

+    Copyright (C) 2024 Oliver T. Hofmann
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License as
+    published by the Free Software Foundation, either version 3 of the
+    License, or (at your option) any later version.
+
+    This program 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 Affero General Public License for more details.
+
+    You should have received a copy of the GNU Affero General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
+    If you are using this software for scientific purposes, please cite it as:
+    L Hoermann et al., Computer Physics Communications (2019), 143-155