Add time limits to Nodes

To end the Node at a particular time if the agent has not generated an Action, simply set the Sensor's duration_msec field to an integer value.

Example of adding a time limit to a Node 
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import nodekit as nk

# The Node's KeySensor will time out with a WaitAction after 5000 msec:
node = nk.Node(
    card=nk.cards.TextCard(text='Hello, world!'),
    sensor=nk.sensors.KeySensor(
        keys=['f', 'j'],
        duration_msec=5000 
    )
)

Note that by default, the Sensor's duration_msec is set to None, which means the agent has an unlimited amount of time to generate an Action for the Sensor.