Core API: index properties after the fact

[jri]

For topic/assoc values there is support for after-the-fact-indexing already.
The same is required for node properties.

[Jörg Richter <jri@…>]

In 420c62145a8bd307d56f0b574209cc2467d0507b/deepamehta:

Core API: add property indexes afterwards (#897).

2 new methods in DeepaMehtaService:

void addTopicPropertyIndex(String propUri)
void addAssociationPropertyIndex(String propUri)

See #897.