Retrieves relation object between user and segment. user and segment may be null. In such case this parameter is
ignored and paginated collection of relations is returned.
Parameters:
user - defining one side of relation - may be null which means all users
segment - defining other side of relation - may be null which means all segments
baseSite - defining other side of relation - may be null which means all segments,users
pagination - holder of pagination data and sort options
Returns:
collection of relation between given user and segment. Empty if no relation was found.