Add test for `cost_function.py`

This test requires a specially prepared file `segmented.gt`:

```
head -n 100000 /data/guez/Oceanic_eddies/Global_1993_2020/Eddy_graph_anti/edgelist.csv >edgelist.csv
~/Detection_eddies/segments.py edgelist.csv segmented.gt
```

because we need input with `%Y/SHPC_(anti|cyclo)` arborescence for the
test.