v		input view
cols		column references
gcol		new group column name

Group takes a view v and rearranges all repeated values in cols, so that they appear once, with all other columns grouped into a new gcol subview column.

If cols is empty, all rows will be moved into a single subview.

set v [view {A B} def {a 1 b 2 c 5 b 3 b 4}]

view $v group A G

	A	G
0	a	B 0 1
1	b	B 0 2 1 3 2 4
2	c	B 0 5

view $v group {A B} G

	A	B	G
0	a	1	0
1	b	2	0
2	c	5	0
3	b	3	0
4	b	4	0

view $v group {} G

	G
0	A B 0 a 1 1 b 2 2 c 5 3 b 3 4 b 4