Improved get_unabbreviated_triples()

[jesper-friis]

Description:

• Corrected behaviour of get_unabbreviated_triples().
• Changed add_label_annotation() to create missing label annotations instead of raising an exception.
• Minor cleanup

PR added to support the "ontopy" triplestore backend in DLite.

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments:

[francescalb]

Please write a proper issue first describing the problem. this then makes it easier to review why and how this is implemented.