Need a mechanism for linked instances that are not resolved

[ehennestad]

Add a (hidden/protected) property on the instance indicating whether the instance is resolved.

This could be assigned automatically if an instance is initialised with only an id.

This property would mostly control the display of an instance, but could potentially also be used to issue warnings if an instance is retrieved, but not automatically resolved.

Two use cases:

1. A local collection, in which case instances should be resolved when the collection is initialised.
2. A remote collection, i.e the EBRAIN Knowledge Graph, where instances might be resolved on demand.