SpatiocyteWorld.actual_volume changed its meaning

See https://github.com/ecell/ecell4-base/issues/254

```
util/ports.py: volume = w.volume() if not isinstance(w, ecell4_base.spatiocyte.SpatiocyteWorld) else w.actual_volume()
util/simulation.py: volume = w.volume() if not isinstance(w, ecell4_base.spatiocyte.SpatiocyteWorld) else w.actual_volume()
```