Add DenseVector std::initializer_list constructor
Add DenseVector std::initializer_list constructor

Most of the examples I saw online had the initializer_list being
passed by value, so I have copied that approach here.

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