Unexport two symbols that don't need exporting

Neither of these implementation details should be part of the API:
- find-triples-with-subject-uuid
- delete-triples-with-subject-uuid
Update find/delete-triple functions

Extend 'delete-triples-with-subject-uuid to check whether there _are_
any triples to delete before trying to delete them.

Add two functions to find and remove triples with a given object value:
- find-triples-with-object
- delete-triples-with-object
Improve the comment for 'extract-uuid-from-string
Add and export 'delete-narrative-version
Add 'get-all-narrative-versions