* Improve handling of NaN in NumberTransformer and CatTransformer.
* Add unittests for HyperTransformer.
* Remove unused methods `get_types` and `impute_table` from HyperTransformer.
* Make NumberTransformer enforce dtype int on integer data.
* Make DTTransformer check data format before transforming.
* Add minimal API Reference.
* Merge `rdt.utils` into `HyperTransformer` class.