* Creates an association between two epics, designating one as the parent epic and the other as the child epic. A parent epic can have multiple child epics. If the new child epic already belonged to another epic, it is unassigned from that previous parent.
*
* <pre><code>GitLab Endpoint: POST /groups/:id/epics/:epic_iid/epics/:child_epic_id</code></pre>
*
* @param groupIdOrPath the group ID, path of the group, or a Group instance holding the group ID or path
* @param epicIid the Epic IID to assign the child epic to
* @param childEpicId the global ID of the child epic. Epic IID can’t be used because they can conflict with epics from other groups.
* @return an ChildEpic instance containing info on the newly assigned child epic
* @throws GitLabApiException if any exception occurs