docs(pm4py): added missing license headers

pm4py/util/nx_utils.py

@@ -1,3 +1,19 @@
+'''
+    This file is part of PM4Py (More Info: https://pm4py.fit.fraunhofer.de).
+
+    PM4Py 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.
+
+    PM4Py 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 PM4Py.  If not, see <https://www.gnu.org/licenses/>.
+'''
 import networkx as nx
 from enum import Enum
 from pm4py.util import exec_utils, constants, dt_parsing