Extend reference osm structure and add comments.

Add qHash function for osm reference. Now it can be used in sets and as hash key. Add constructors to osm reference to simplify casting from full structures.
Change osm relation member to contain reference field instead of separate id/type.
Change variable naming to be more clear.
Change iRelMember to iMember.
Add method for checking presence of data in data storage based on entity reference.