Rename radii and rossby to `avg_rad` and `avg_Rossby`.

Add tests for a case where the speed is not defined on a whole segment.

Move tests `Plot_eddy_contours_Mat` and `Stats_Matlab` to
`Convert_Matlab/Tests/long_tests.json`. Thus, we group the tests that
depend on the availability of Matlab.

Loop on nodes if the number of nodes is small compared to the number
of shapes. For faster execution.

Instead of a file name. Preparing for some more elaborate processing
in `set_attribute`.

Rename `first_SHPC` and `last_SHPC` to `i_SHPC`. Rename `first_loc`
and `last_loc` to ishape.

This will allow to see repeated code and use fewer variables.

The modification of `sys.path` was added in commit 034d96fe to have
access to `util_eddies`. It is no longer useful, since commit
d16d3f1c, when a symbolic link was added.

Return tuple or scalar from function `node_to_date_eddy`, depending on
new argument `only_date`, instead of returning a dictionary. The
function is easier to use. Also, it saves a useless computation if we
are only interested in the date index. Returning a dictionary was more
justified before commit 43d2eff6, we we also returned the date.

Simpler, no subtraction.

Return from function `calculate_radii_rossby` the average radius and
Rossby number instead of the sum of average radii and the sum of
Rossby numbers.

Since the length of the segment is lower than or qual to
`num_of_days_to_avg`.

Pass a restricted list of eddies to `calculate_radii_rossby`: the list
over which to calculate mean Rossby and radius. Instead of passing the
whole segment and the indices of eddies in the segment.

This allows readability of the property in graphml format. As vector
of integers is more specific than python object, I suspect this commit
also makes the code more efficient.

For easier comparison.

Preparing to introduce an option.