Update authors and tidy imports.

tools/outputfiles_merge.py

@@ -1,5 +1,5 @@
 # Copyright (C) 2015-2021: The University of Edinburgh
-#                 Authors: Craig Warren and Antonis Giannopoulos
+#                 Authors: Craig Warren, Antonis Giannopoulos, and John Hartley
 #
 # This file is part of gprMax.
 #
@@ -25,7 +25,7 @@ from pathlib import Path
 import h5py
 import numpy as np
 from gprMax._version import __version__
-from gprMax.utilities import natural_keys
+from gprMax.utilities.utilities import natural_keys
 
 logger = logging.getLogger(__name__)