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.

Maybe it would be better to have separate method for checking the state? To me it's would not be transparent (without reading the documentation) that this is the expected behavior from passing `None`. I'd prefer method, because that would allow me to figure out what going on simply by looking at the function names.