Draft: Full matcher with preview and maybe PPU support

If we need all frames for a given train in #23 (closed), data flows get messy one way or another. One option would be to generate the preview in the place in the topology where one might place a full matcher.

As stacking is part of preview assembly (and the ShmemTrainMatcher already has code to handle configuration of stacking indices and such), an actual full matcher could just be given extra preview outputs. This is essentially making !10 (closed) (adding multi-frame support to DetectorAssembler) redundant. Side note: I have been considering axing DetectorAssembler / reusing the stacking approach for it anyway.

With a separate preview output, we could decide to only send certain trains on this channel (NYI). For full data, that is obviously necessary. Have not tested how the preview looks; it crashes my karabo-gui when I connect (laptop probably has too little memory left).