Fix capacity checks in RouteSplit

[jcoupey]

Issue

Fixes #981, fixes #996.

Tasks

• Refactor to fix regression from Dig into sonarcloud code smells reports #986
• Properly compute sub-route max load values
• Fix capacity validity checks in RouteSplit
• Update CHANGELOG.md
• review

[jcoupey]

So as explained in #981 (comment), it turns out the fix allows to filter out invalid splits, but also to reach to more valid ones. Here is a comparison of results between master (actually the last working commit 95b30e8) and this PR.

Average gap (%) on all usual HVRP instances

x	95b30e8	PR
0	7.55	6.54
1	6.3	5.26
2	5.16	4.36
3	4.66	3.82
4	4.22	3.41
5	4.03	3.25

Computing times are really similar, even a tiny bit lower with the PR. The worst gap is greatly reduced also, regardless of the exploration level, it goes down from 43.54 to 19.17%.

That's a significant improvement! 🎉 This operator was an even better idea than we first thought. ;-)