Support Weights and Improve Time Complexity of Prediction Methods

[vivekjoshy]

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added test(s) covering the changes (if testable)

Lot of changes:

• Implements per-player weights and/or partial play support.
• Support a new balance flag to skew tail end skill ratings in high uncertainty matches. Complements beta parameter.
• Add descriptions to README
• Update dependencies
• Reduce time complexity of all predict methods to $O(n^2)$.

Issue(s) Resolved

Fixes #29

Affirmation

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I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under openskill.py's MIT license.

I certify the above statement is true and correct: @vivekjoshy

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[philihp]

oh snap!! weights!? 👏