fdo#72741 - swap values only inside a formula group.

Speeds up sorting large ranges containing formula groups by a
computational order or two: can sort 1 million rows of =A1 in sub 10s

Change-Id: I729c801e68154986956b1726e1c35ef973cc4248
fdo#72741 - write unit test for in formula group swapping.

Change-Id: I9da62b026b40b08784415e0421556946646d25e2