User interaction with transition tree

[SimonHeybrock]

The data flow in the back end is now based on transitions that move data between checkpoints. The goal of this structure was to support adding and removing of certain reduction parts while running and while incoming events keep updating all checkpoints.

Add some mechanism to add or remove transitions. For example this could we would like to attach a second filter, with a separate result streamer and visualizer (maybe it makes more sense to have only one visualizer that visualizes data from several branches of the transition tree?).

[SimonHeybrock]

For starters, the simplest thing we could try is to add the transitions that are currently in the reducer by default only on demand. That is, by default the reducer would do nothing. Then the user can add reduction and streaming, one by one.

[lottiegreenwood]

Ref in #37