add function that allow to save a geoentity (tetra, triangle,...) as a gmsh mesh file.
In particularly usefull to show the local numerotation of geometric points.
Ex :
    saveGeoEntityAsGMSHMesh(_geoentity=*mesh->beginElement(),_filename="myentityElt.msh");
    saveGeoEntityAsGMSHMesh(_geoentity=*mesh->beginFace(),_filename="myentityFace.msh");
    saveGeoEntityAsGMSHMesh(_geoentity=*mesh->beginEdge(),_filename="myentityEdge.msh");
add boost parameter