Make TNodeXYZ public, which together with SMDS_StdIterator allows getting coordinates...

Make TNodeXYZ public, which together with SMDS_StdIterator allows getting coordinates of all nodes of mesh element in a couple of lines of code:

  typedef SMDS_StdIterator< SMESH_MeshEditor::TNodeXYZ, SMDS_ElemIteratorPtr > TXyzIterator;
  vector<gp_Pnt> coords( TXyzIterator( elem->nodesIterator()), TXyzIterator() );

class SMESH_EXPORT SMESH_MeshEditor
{
public:
+  //------------------------------------------
+  /*!
+   * \brief SMDS_MeshNode -> gp_XYZ convertor
+   */
+  //------------------------------------------
+  struct TNodeXYZ : public gp_XYZ