Coupled soln 2694

I think this coupleSln() is useful, especially for high order elemental variables. Assembling and inverting the mass matrix could be costly. If the variable has the same FE type as the coupled variables, we can directly access the raw solution vector to make the evaluation much more efficiently. Making _nodal_u* in MooseVariable valid does not hurt.

I added a simple accessor for the required quadrature order in MooseVariable. To me, logically the variable is associated with an order, at least for now. A variable has a fixed FE family and order (we do not have p-refinement!). The shape of the element seems to me not affecting the quadrature order as well.