Tidied some import statements.

tools/plot_source_wave.py

@@ -16,7 +16,9 @@
 # You should have received a copy of the GNU General Public License
 # along with gprMax.  If not, see <http://www.gnu.org/licenses/>.
 
-import os, argparse
+import argparse
+import os
+
 import numpy as np
 import matplotlib.pyplot as plt