Geonetworkx is not compatible with `shapely  2.x`

The function `cascaded_union` was removed from shapely 2.x and an error is raised when geonetworkx imports it.

This must be replaced by `unary_union` which is supposed to do the same. The easiest solution is to replace the import in `tools/isochrones.py` and `utils/voronoi_utils.py`:

```
from shapely.ops import linemerge, polygonize, unary_union as cascaded_union
```
