def [](cell_def)
      return rows[cell_def] if cell_def.is_a?(Integer)
      parts = cell_def.split(':').map{ |part| name_to_cell part }
      if parts.size == 1
        parts.first
      else
        range(*parts)
      end
    end
def [](cell_def)
      return rows[cell_def] if cell_def.is_a?(Integer)
      parts = cell_def.split(':').map{ |part| name_to_cell part }
      if parts.size == 1
        parts.first
      else
        range(*parts)
      end
    end
Returns the cell or cells defined using excel style A1:B3 references.
 @param [String|Integer] cell_def the string defining the cell or range of cells, or the rownumber
 @return [Cell, Array]