feat!: move default network into Network()

[tonyandrewmeyer]

This moves the default Network into the Network initialisation, rather than as a separate classmethod that provides a Network object. This is more consistent with the rest of the Scenario objects, which try to have useful defaults where possible.

For the most simple case:

# Previously
network = Network.default()
# Now
network = Network("default")  # The name is needed because of the `State` change elsewhere

To override elements of the default is a little bit more work, particularly if it's in the nested Address object, but it doesn't seem too bad:

# Previously
network = Network.default(private_address="129.0.2.1")
# Now
network = Network("foo", [BindAddress([Address("129.0.2.1")])])