Format instance to dump
        vertices is dict of name to Vertex

def format(self, vertices):
        """Format instance to dump
        vertices is dict of name to Vertex
        """
        index = ' '.join(str(vertices[vn].index) for vn in self.vnames)
        vcom = ' '.join(self.vnames)  # for comment
        return 'hex ({0:s}) {2:s} ({1[0]:d} {1[1]:d} {1[2]:d}) '\
               '{4:s}  // {2:s} ({3:s})'.format(
                    index, self.cells, self.name, vcom, self.grading.format())