Feature/restart finished demultiplexing

- Implemented functionality for removing the "found" file for processing units (a.k.a. "forgetting" the demultiplexing of that unit)
- Went over the state change events in SisyphusMaster and made them consistent
- Wrote tests for the IlluminaProcessingUnit[Executor]Actor to complement the forget functionality 
- Added a "ProcessingUnitPlaceholder" class, which can be passed to a ProcessingUnitWatcher and eventually exchanged for a real ProcessingUnit if it is available to the watcher.

I actually hadn't seen `headOption` before - but it looks highly useful, so I'm going to start to use it. :smile: