code: def apply_format(self, fmtfct):
          
          for ridx in range(self.nrows):
              for cidx in range(self.ncols):
                  # MUST set the parent as local view is immutable
                  riloc = self.row_ilocs[ridx]
                  ciloc = self.col_ilocs[cidx]
                  self.parent.formatted_values.iloc[riloc, ciloc] = fmtfct(self.actual_values.iloc[ridx, cidx])
          return self
docs: For each cell in the region, invoke fmtfct(cell_value) and store result in the formatted_values
          :param fmtfct: function(cell_value) which should return a formatted value for display
          :return: self