Fix type hints from H3Index to H3IndexInput

[4nthonylin]

Noticed that cellToBoundary was type hinted to accept H3Index however it actually can accept H3InputIndex. Updated type hints for cellToBoundary and a few other functions I noticed that had the same problem.

[coveralls]

Pull Request Test Coverage Report for Build 3960695796

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.844%

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Totals
Change from base Build 3941620029:	0.0%
Covered Lines:	529
Relevant Lines:	529

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💛 - Coveralls

[nrabinowitz]

Thanks for contributing!