Introduce std::reference_wrapper in fem_system_ex1
Introduce std::reference_wrapper in fem_system_ex1

Similar to the change in fem_system_ex3 in #2513.

jwpeterson/libmesh,dschwen/libmesh,BalticPinguin/libmesh,libMesh/libmesh,jwpeterson/libmesh,libMesh/libmesh,BalticPinguin/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,BalticPinguin/libmesh,libMesh/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,roystgnr/libmesh,roystgnr/libmesh,dschwen/libmesh,jwpeterson/libmesh,roystgnr/libmesh,dschwen/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,roystgnr/libmesh,dschwen/libmesh,jwpeterson/libmesh,dschwen/libmesh,jwpeterson/libmesh,libMesh/libmesh,roystgnr/libmesh,jwpeterson/libmesh,roystgnr/libmesh,libMesh/libmesh,BalticPinguin/libmesh,libMesh/libmesh,BalticPinguin/libmesh,libMesh/libmesh,dschwen/libmesh