Remove intermediary variables `first_term`, `second_term`, `third_term`.

On second thought, I find the default value confusing. We are
reverting commit b328ab6f.

Add arguments `input_segments` and `output_segments`. Because on
one occasion I wanted to use and keep two versions of input and output
graphs in the same directory

Note that orientation = None is passed to `get_ishape_last` where it
becomes `self.def_orient`.

Move function `comp_ishape` to method of SHPC class.

`slice` is a built-in class of Python.

The global variable was used in the body of the function.

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.