An S4 class to represent a permutation order.
rowsAn integer vector giving the rows permutation.
columnsAn integer vector giving the columns permutation.
methodA character string indicating the seriation
method used.
x[[i]]Extracts informations from a slot selected by
  subscript i. i should be one of "id",
  "rows", "columns" or "method". Any unambiguous
  substring can be given.
Other class: 
DateEvent-class,
DateMCD-class,
DiversityIndex,
IncrementTest-class,
RefineCA-class