Merge pull request #4 from collective/merged_relations

Merged forward and backward relations
Refs #17059; Added auto relations to the relations macro merge:

- Added auto relations to the relations macro merge at it can also have
  the same label as a forward or backward relation
- Return an empty list from the auto relations instead of a boolean to
  be consistent with the way the other relations methods return empty
  values
- Removed duplicates of the resulting list extension by passing them
  through a set
Merge remote-tracking branch 'collective/master'