Unable to represent a listed FutureContract and a specific purchased FutureContract

[rivettp]

I'm guessing that the class https://spec.edmcouncil.org/fibo/ontology/DER/ExchangeTradedDerivatives/Futures/Future represents a specific held contract, acquired (possibly with others) on a specific date by a specific owner, rather than the exchange listing with the standardized terms. Need to have a property to link it to the latter, and the ability to even model the latter.

BTW why "Future" and not "FutureContract"?

Originally posted by @rivettp in #990