[license] add GPL license header to python tool to make things more clear

gen_png.py

@@ -1,5 +1,22 @@
 #!/usr/bin/env python3
 
+###########################################################################
+#    Copyright (C) 2022  Stefan Riesenberger
+#
+#    This program 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 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 General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+###########################################################################
+
 import glob
 import ntpath
 import subprocess