def insert_row idx, cells=[]
      res = @rows.insert idx, Row.new(self, idx, cells)
      updated_from idx
      res
    end
def insert_row idx, cells=[]
      res = @rows.insert idx, Row.new(self, idx, cells)
      updated_from idx
      res
    end
Insert a Row at _idx_ (0-based) containing _cells_