[Feature Proposal] Change instance methods to static methods

[Apricot-S]

Description

Some methods that are currently instance methods are essentially pure functions, so we propose to turn them into static methods.
These include:

• Agari.is_agari()
• Shanten.calculate_shanten()
• Shanten.calculate_shanten_for_chiitoitsu_hand()
• Shanten.calculate_shanten_for_kokushi_hand()
• Shanten.calculate_shanten_for_regular_hand()
• HandDivider.divide_hand()
• HandDivider.find_pairs()
• HandDivider.find_valid_combinations()
• FuCalculator.calculate_fu()
• FuCalculator.round_fu()
• HandCalculator.estimate_hand_value()
• ScoresCalculator.calculate_scores()
• Aotenjou.calculate_scores()
• Aotenjou.aotenjou_filter_yaku()

Pros

• Eliminates the need to create instances for calculations.
• Since static methods can also be called from instances, there is basically no need to modify existing code.

Cons

• This may result in a slight slower execution speed, since local variables instead of fields are created and destroyed each time.
• This is a breaking change because some classes, such as Shanten, have class variables defined without a leading underscore.

[Nihisil]

I don't mind this improvement, it looks good to me. Performance isn't the top priority for this lib because of Python, and for people who want faster solutions there are Rust and C versions of mahjong libs on GitHub

[Apricot-S]

for people who want faster solutions there are Rust and C versions of mahjong libs on GitHub

By the way, I've also developed a Rust library xiangting for shanten number calculation, as well as a PyO3 binding xiangting-py.
They use a dynamic programming approach and aim to provide one of the fastest implementations available, while still being accessible from Python.