Adding ScalarCoupleable interface to ElementUserObjct (closes #3265)

It was not able to use scalar variables in anythong derived from
ElementUserObject, because it was missing ScalarCoupleable interface.
Merge pull request #3266 from andrsd/scalar-coupleable-for-elemental-user-objects

Adding ScalarCoupleable interface to ElementUserObjct (closes #3265)
Merge commit '112d8fc4b925e6befd5c76bd147a3b9be4ad9cf9'