Add LinearLinkageEncoding

[abeham]

It is a better suited encoding for grouping problems ​link.

The idea is that a vector holds references to other items in the vector, e.g.

Item#	1	2	3	4	5	6	7	8	9
Link	2	4	6	9	7	6	8	8	9

This identifies the following three groups:

1. { 1, 2, 4, 9 }
2. { 3, 6 }
3. { 5, 7, 8 }

The advantage over a numbered encoding of groups (e.g. an integer vector that assigns a group number to each item) is that there are no ambiguities regarding a change in the group numbers. Typically, for anonymous groups, exchanging all items of group 1 to be group 2 and those of group 2 to be group 1 would not create a different solution. But in e.g. a population-based approach these would be considered as different genes.

To avoid further ambiguities, the link for an item must always be greater than or equal to its index. And with the exception of the ending items (where item# equals link) no number must occur more than once as link.

There are different variants:

• LLE is the variant described above
• LLE-b is the same, but with backward links instead of forward links
• LLE-e uses the number of the highest element in the group as group number

[abeham]

r12285: Added branch for linear linkage encoding (LLE-e)