Add a method to retrieve a single ancestry for terms with multiple parents

Problem/Motivation

When a term has multiple parents, the return value from TermStorage::loadAllParents() does not contain enough information to make out a proper ancestry (it just contains all possible parents).

This issue is demonstrated in the SHS module ( #2288035: Doesn't work properly with terms that have multiple parents → ), where a separate helper function will be used to determine a single ancestry.

Proposed resolution

Add a new method to TermStorage (or alternatively, a second parameter to the loadAllParents()) to retrieve the first ancestry found for a term with multiple parents.

Remaining tasks

User interface changes

API changes

Data model changes