Simplify Gene Pool stat tracking
Simplify Gene Pool stat tracking

1. Replace std::map<size_t, X>, where the size_t key is the index of the
gene pool, with std::vector<X>, since this has the same structure as the
gene pools themselves.

2. Instead of erasing deead AIs from the stat containers, make a new
container and copy over the entries from still-living AIs.

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