Fixed a bug in the timeline group generation: Labels for groups were previously taken directly from the field acting as primary key, which made sense. But with removing the primary key of the specific fields, the labels turned into the numeric IDs, which was not useful. Had to introduced two additoinal parameters to the timeline group constructor __init__ to make sure that the timeline can find the proper labels. This has to be properly fed by the function generating the timelinegroup. (i.e., the views)