Split constituent writer class into another package

General idea is to make a more generic writer class that will deal with truth particles, tracks, subjets, and (eventually) pflow constituents. I restructured stuff a bit to accommodate that. I'm hoping that in doing so we can remove some repeated code, and also make the classes we do have more flexible (right now, for example, a lot of them don't let you change which variables are written).

This merge request only implements the track writer in the generic format (I assumed that would be the hardest one). I also removed BTagTrackWriterConfig and used the generic class config instead.