Gap-stat

Added
- Optimize Rust backend
- Benchmark setup
- Create a separate CHANGELOG

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Added
- Add OSX support to CI/CD [31](https://github.com/milesgranger/gap_statistic/pull/31)
Changed
- Fix passing of null dataset in python gap value calculations [34](https://github.com/milesgranger/gap_statistic/pull/34)
- Use advanced method for choosing k, documentation updates, plotting, other enhancements. [28](https://github.com/milesgranger/gap_statistic/pull/28)
- Change license to dual Unlicense / MIT [30](https://github.com/milesgranger/gap_statistic/pull/30)

Added
- Return the reference distributions' standard deviation in the dataframe results

Added
- Support user defined functions for the clustering algorithm used in the gap statistic process
- Migrate to Azure Pipelines C

Changed
- Fix calculation of gap statistic

Added
- First stable release with Rust backend. OptimalK now takes `backend="rust"`
- More tests, add best init in rust kmeans impl for better stability between runs