Include an overall summary number in compare output

People often quote a single perf number when comparing versions (e.g. "Python 3.11 is 25% faster than Python 3.10")

The pyperformance compare command prints out the "change factor" per benchmark. It's not clear how to go from this list of numbers to a single overall number. Having the compare command compute and print it out would also help with consistency when quoting this single perf number (is it mean? geometric mean? only including significant results or all of them?)

まず pyperformance compare コマンドの場所を特定し、現在どのようにベンチマークごとの変化係数と有意性を報告しているかを確認します。メンテナーと集計ルールを明確にし、完了条件を、compare が文書化された全体サマリーの数値を一貫して出力することと定義します。