Move StreamFunction utility to a FieldConvert module
This removes the StreamFunction2D extra utility, replacing it by a FieldConvert module.
I am sure it this works very well because of the boundary conditions, so I just did exactly what is done in StreamFunction2D and included a note on the user guide about the boundary conditions.
After this, Extras will have only CheckXmlFile (which may be obsolete with NekMesh?) and 3 vtk utilities (2 of which are pure vtk with no Nektar++ code).