[Persistence_matrix] Rows not updated when two columns from different matrices are swapped.

[hschreiber]

Were problem can arise when swapping two columns from different matrices, is at the row access, as the cells in the rows will not be swapped (you should first unlink a column and relink it after swap). [...] Nevertheless, I realized that I never mentioned this in the doc. I could also just add the linking/unlinking in the swap operator.

Originally posted by @hschreiber in #669 (comment)