Added choices help to command line arguments.

tools/plot_Bscan.py

@@ -28,7 +28,7 @@ from gprMax.exceptions import CmdInputError
 # Parse command line arguments
 parser = argparse.ArgumentParser(description='Plots a B-scan image.', usage='cd gprMax; python -m tools.plot_Bscan outputfile output')
 parser.add_argument('outputfile', help='name of output file including path')
-parser.add_argument('output', help='name of output component to be plotted (Ex, Ey, Ez, Hx, Hy, Hz, Ix, Iy or Iz)')
+parser.add_argument('output', help='name of output component to be plotted', choices='Ex, Ey, Ez, Hx, Hy, Hz, Ix, Iy or Iz')
 args = parser.parse_args()
 
 # Open output file and read some attributes