Related Thing, Datastream, Observation, Feature

[Hylke van der Schaaf]

Self-relations for the EntityTypes Thing, Datastream, Observation and Feature are quite common.
Like #65, and the RelatedDatastream concept of STAplus.

The current proposal (#167) adds a Relations Extension:

Open Question: do we need a time (TM_Object or TM_Instant) on these relations?