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.

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.

Preparing to introduce an option.

Rename function `calculate_radii_and_rossby` to `calculate_radii_rossby`.

Remove intermediate variables `first_pos` and `last_pos`.

Rename property `nl_cost_function` to `cost_function`.

Useless in this script.

Simply printing g displays the address of the graph, which may change
between runs. This was not convenient for the comparison of runs.

To ease comparison of runs.