get_referenced method were implemented for SingleReference and MultipleReference class
They are able to return LeObject instances refereced by them

Deleted old methods that are unused (one of them was triggerring an error will doing nothing)

**kwargs was not given to super() constructor

Test passes

In construct and check data

Return the value of this datahandler when empty

refs #131

refs #131

Methods are written.
They are not called by anyone and the only tests that were done is that no syntax error is raised.
( refs #131 )

- deletion of unused attributes
- copy the back_reference checks in _set_back_reference function and use it in Reference.__init__()