add_named_listener: Add named listener to an event
In mrc-ide/safir2: Individual-Based Model of COVID Transmission

Given a individual::Event or individual::TargetedEvent object, add a named listener to the list of listeners stored in that event.

1	add_named_listener(event, name, listener)

`event`	a individual::Event or individual::TargetedEvent object
`name`	a name
`listener`	a function which takes either a single argument (`timestep`) if event is a individual::Event or two arguments (`timestep`, `target`) if the event is a individual::TargetedEvent