Add support of selecting different from subsets proportionately (#270)

* Use numpy to count the unique number of labels for efficiency

* Add support of selection different cluster subsets proportionately

* Add tests for the proportional selection

* Fix the problem of when zero number of elements selected from the minority class

* Add tests for imbalance case of multiple classes

* Add testing data for imbalance cases

* Reformat with black

* Add data points to smallest cluster when not enough data points

* Add test for checking the number of labels match the number of total data points

* Ignore NotImplementedError in coverage report

* Add typing hints for returns

* Add typing hints

Update README.md (#265)

* Refactor the README.md

* Add installation section to intro.md

Fix the building reference index when `ref_index` is None in `DISE` (#…

…264)

* Fix the return type when calculating medoid in `get_initial_selection`

* Add test case for ref_index is None

* Remove redundant checking of ref_index

Update logo position

Rename yml --> yaml

Adjust the logo width

Fix text rendering in README.md

Update README.md in pyproject.toml

Update ignore list

Update version number