2D array describing each registered dimension
        together with headers - for use in __str__

def _gen_dimension_table(self):
        """
        2D array describing each registered dimension
        together with headers - for use in __str__
        """
        headers = ['Dimension Name', 'Description',
            'Global Size', 'Extents']

        table = []
        for dimval in sorted(self.dimensions(copy=False).itervalues(),
                             key=lambda dval: dval.name.upper()):

            table.append([dimval.name,
                dimval.description,
                dimval.global_size,
                (dimval.lower_extent, dimval.upper_extent)])

        return table, headers