ModularServer: server port should be specifiable during __init__

[rht]

In the examples, currently it looks like this

server = ModularServer(
    BoltzmannWealthModelNetwork, [grid, chart], "Money Model", model_params
)
server.port = 8521

It would be cleaner as

server = ModularServer(
    BoltzmannWealthModelNetwork, [grid, chart], "Money Model", model_params, port=8521
)