Make the program test_set_all_outerm more to the point: testing the

procedure set_all_outerm. So do not define and output null max-speed
contours.

Bug fix in program test_set_all_outerm: take into account missing
values in ADT.

Make plot_snapshot.py work even if there is no max-speed contour
shapefile. So we can use plot_snapshot.py on results of program
test_set_all_outerm.