Add multiple properties fetch to RdfSource

[skodapetr]

As of now RdfSource allow use to read one property at a time. When reading multiple properties for a single object such approach is sub-optimal.

In addition we can also add new methods that allow user to request multiple properties, e.g.

properties(iri: string[], predicate: string): Promise<{[predicate:string] : RdfObject[]}>;

and similar for reverse property.

The default implementation may utilize the property and reverseProperty operations. In the final step an optimization can be implemented for SparqlSource.

[sstenchlak]

This is still partially relevant. We have a concept of views, where the user may request a surrounding. But there is no mechanism to fetch multiple surroundings at once or numerous single resources at once. Nevertheless, due to the current set of "CIM adapters", there is no case where this would be beneficial.