Improve 2-opt

[jcoupey]

In the symmetric case, trying the move with edges (e_2, e_1) is the same as with (e_1, e_2). So better stop at some point to avoid testing pairs in both orders. This would improve timing on all problems (as even asymmetric instances use this at some point) and might be a huge speed-up for big instances, where local search is the most time-consuming phase.

The change should be made with caution as:

• the two loops should go through the edges in the same order to easily avoid duplicates, without having to remember every pair;
• in the inner loop, going through the edges in the order of the current tour IS mandatory for efficient computation of the cost of the part that has to be reversed (asymmetric case).