hooks for the runner

[timziebart]

For the next dev-treff: I would suggest, we introduce a mechanism so model components, models and scripts can register functions ("hooks") that are called:

1. at runner startup
2. when the runner stops inbetween, e.g. for a step or an event
3. at runner end

Reasons are:

1. sometimes analysis before the run have to be done, e.g. the small speed-up for the adaptive voter model needs to analyze the graph at start-up
2. for active plotting
3. clean-up procedures, e.g. free ram

[timziebart]

78982b6d8d8a5a2e7fc6e759ba766c98fae901b0 done

[timziebart]

still, they need to be added to the runner scheme