Update authors and tidy imports.

gprMax/sources.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.
 #
@@ -22,7 +22,7 @@ import gprMax.config as config
 import numpy as np
 
 from .grid import Ix, Iy, Iz
-from .utilities import round_value
+from .utilities.utilities import round_value
 
 
 class Source: