Clarify Definition of Timestamped Graph Re Multiplicity of Timestamp Triples about rsp-ql HOT 5 CLOSED

The main advantage if allowing a timestamped graph to have more in its default graph than just the timestamp triple is that these triples would be selected, along with the timestamp triple and the named graph, when a window function is applied. The other mechanisms for including this information in the stream would not allow so convenient method of selection.

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To make this more general notion work, a timestamped graph cannot be "just" an RDF dataset, because an RDF dataset does not have a mechanism for distinguishing a particular triple of the default graph as the timestamp triple. There may be more than one that meets the essential requirements ( e.g. subject is the name of the named graph)

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I have managed to write the definitions for things like isomorphism and stream merge/union so that it doesn't matter if there is more than one triple in the default graph of a timestamped graph, as long as there is only one that is designated as the timestamp triple. It should be possible also with the semantics. So I think this clears the way to allow multiple triples in the default graph of a timestamped graph.

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In the pull request, there is further clarification about the nature of temporal entities, in that they don't need to be literals. I believe this can allow handling the case where the stream order is specified, in linked-list manner, by having an "after" timestamp predicate.

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resolved by #40

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