def set(row, col, value, sheet = nil) #:nodoc:
      key = normalize(row, col)
      cell_type = cell_type_by_value(value)
      sheet_for(sheet).cells[key] = Cell.new(value, cell_type, nil, cell_type, value, nil, nil, nil, Coordinate.new(row, col))
    end
def set(row, col, value, sheet = nil) #:nodoc:
      key = normalize(row, col)
      cell_type = cell_type_by_value(value)
      sheet_for(sheet).cells[key] = Cell.new(value, cell_type, nil, cell_type, value, nil, nil, nil, Coordinate.new(row, col))
    end
set a cell to a certain value
 (this will not be saved back to the spreadsheet file!)