Improve shuffle by SR algorithm

[ETLegacyTeam]

Author: @rmarquis (Spyhawk)
Date: 2019-05-21
Redmine Issue: 1299, https://dev.etlegacy.com/issues/1299

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Shuffle by SR algorithm is implemented in a simplistic way (much like XP sort), but it might be possible to improve it by using mu and sigma directly instead of the rating.
Investigate and improve if possible.

Use the TS game quality function. Compute all permutation, or find a way to find one that is "good enough" quickly if that is too costly. Keep the constraint of equal number of players per team (and put the extra odd player in the weaker team), take map bias into account. Maybe add an option to give a closer game quality but with more unbalanced team (see etpub rating balance)?

See f.e.:

• https://forums.faforever.com/viewtopic.php?f=41&t=12862
• https://en.wikipedia.org/wiki/Heap%27s_algorithm for fast permutation

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Related commit(s):
164876b - mod: merged shuffle XP and SR commands, refs 403 ..