weight. So, in procedure set_max_speed, in case radius equals 1, set
e%max_speed to the speed on the outermost contour instead of zero.

In procedure set_max_speed, in case the highest speed is on the
outermost contour, set e%speed_cont%polyline to null_polyline instead
of e%out_cont%polyline. This saves main memory and there is less
copying to do.

speed_cont can now be null even if max_speed is not zero so change
test for null shape in procedure write_eddy.

It appears that the shapelib C library cannot write a NaN value to a
dbf file, so write 1e4 instead.

Bug fix in plot_snapshot.py.