EDIT

Currently the highes level of units in the derivative tree editor is the field unit. However, in the model and in reality field units sometimes share the same GatheringEvent (currently holds data on where, who and how, maybe extended in future #9445).
We should therefore extend the specimen tree editor in the way that GatheringEvent may become the root element.

This functionality should be configurable 1.) by CdmPreferences and 2.) by selection in the search UI (e.g. by a checkbox). The CdmPreferences should define the default value for the searchUI configuration value. The default value for the CdmPreference should be not to show the GatheringEvent (to keep the UI simple for default usecases, and for now as this may change in future).