Change and add license texts to apply for GPL3

Source/PS_Source/StretchSource.cpp

@@ -1,3 +1,21 @@
+/*
+
+Copyright (C) 2017 Xenakios
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of version 3 of the GNU General Public License
+as published by the Free Software Foundation.
+
+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 (version 3) for more details.
+
+www.gnu.org/licenses
+
+*/
+
+
 #include "StretchSource.h"
 #include <set>