Rename `get_SHPC` to `get_slice` and `i_SHPC` to `i_slice`.

Assume that a slice is in a directory
"{shpc_dir}/{orientation}/Slice_{slice}" where shpc_dir can be any
name, orientation is "Anticyclones" or "Cyclones" and slice is an
integer. Do not read or create any longer a file "orientation.txt" in
the slice directory. Add procedure `get_slice_dir`. Add dummy
arguments cyclone and slice to procedure `shpc_open`. Add dummy
argument slice to procedure `shpc_create`.

Use `e_overestim.txt` instead of `node_id_param.json`. Since commit
43d2eff6, we wrote only `e_overestim` to the JSon file. Simpler to
write the value alone in a text file.

The maximum speed contour may not be defined separately from the
outermost contour. So if the radius from the maximum speed contour
shapefile is not defined, use the radius from the outermost contour
shapefile.

Motivation: performance. We eliminate repeated reading of the same
information from the shapefiles. If the length of the segment is
smaller than 14 days then we re-use for the last part of the segment
some of the properties from the first part. Also, we do not compute
properties for the first part of the segment if the segment has no
predecessor and for the last part of the segment if the segment has no
successor.

Remove intermediary variables `avg_rad` and `avg_Rossby`.

We extract part of function `calculate_radii_rossby` into new function
`node_to_prop`. So we now separate reading data from the shapefiles
and computing the average radius and Rossby number. Thus, we prepare
for rewriting the algorithm in the main part of the script. The
quantity of reading is still the same, for now.

Rename variable `num_of_days_to_avg` to `n_days_avg`.

This allows readability of the property in graphml format. As vector
of double is more specific than python object, I suspect this commit
also makes the code more efficient. See also commit c7f7fd46.

Return a tuple from function `calculate_radii_rossby` instead of a
dictionary.

The vertex property `avg_Rossby` was never set after `n_valid_Rossby ==
0`. It appears that it had then a default value of 0. Better to
explicitly set it to 0.

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

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`.