[cmake] improve python script for creating CMakeLists.txt

bin/util/create_cmakelists.py

+# Create files CMakeLists.txt for the current folder and all subfolders.
+# Should be run from the folder `dumux` that contains the header files, namely,
+# one level below the top dumux folder:
+# python ../bin/create_cmakelists.py.
+
 # Import the os module, for the os.walk function
 import os
-import re
 
 # Set the directory you want to start from
 rootDir = '.'
@@ -30,4 +34,5 @@ for folderName, subFolders, files in os.walk(rootDir):
                 cmakelists.write("%s\n" % fileName)
 
         cmakelists.write("DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dumux/%s)\n" % folderName[2:])
-        cmakelists.close()
+
+    cmakelists.close()