Support user defined clustering functions

• User can pass in their own clustering function into OptimalK (fe1b1d3)
• Switch to Azure Pipelines (9020d95, db030ee)
• Use black formatting for the project (bdb4af1)

Fix Gap statistic calculation

Merge pull request #15 from milesgranger/fix-gap-calculation

Fix gap value calculation, up to 1.5.2

Stable Rust backend added

Release contains an initial stable release of the same API found in the first release, along with OptimalK taking an additional value "rust" for the backend parameter.

Rust backend - Alpha

Initial release including a Rust backend implementation of the gap statistic & kmeans.

Initial public release

Support for Joblib, Multi-processing and single core