idgears: Try many guesses and select best kmeans-fit.

* Add test-file with 4 engine-cycles.
idgears: Restructure and finalize names

Add tests.
docs: Add `idgears` into autodoc generation.
idgears: Add tests.

- Split ratio-detection from gear-identification.
idgears: New minor func for gear-ident from cycle-data, with test.

test,ci: Add idgears into tests & coverage.