spherical_polygon_area to spher_polygon_area.

In procedure max_speed_contour_ssh, instead of aborting when the speed
is not defined, continue with missing_ssh. Found such a case in
November 29th, 2015 data. Added corresponding test. In procedure
set_max_speed, if max_speed_contour_ssh returns missing_ssh then we
just take the outermost contour as max-speed contour.

script distribution_function.py. Return dictionary instead of tuple:
easier to use.

In script plot_snapshot.py, move definition of readers to the
beginning so there is no risk of entering the window for nothing if we
forgot the end_filename option. Also, enter the four values defining
the window in a single input: easier to copy and paste.

and test_set_all_outerm.

procedure get_eddy), with corresponding test.

Take into account the fact that derived type polygon became polyline
in library Contour_531.

Use new procedure null_polyline from library Contour_531.

Correct a sneaky bug in mean_speed: the last point in the polygon
array is a duplicate of the first point, do not take the last point
into account when averaging (and interpolating). Discovered this bug
because the mean speed was not exactly the same for equivalent
polygons with circular permutation.

Change philosophy for the tests: read whole NetCDF variables, prepare
different NetCDF files for different geographical regions if needed;
read all data from file or namelist; read names of files on command
line.

Use generic test_program.sh (which is out of this project) instead of
the specific tests_detection_eddies.sh.