text: Title: Minor class in multi-class classification with too few samples to stratify
  Body: In the case of too few samples to perform stratification there is an error thrown:
  
  ```py
  The least populated class in y has only 2 members, which is less than n_splits=5.
  ```
  
  Maybe we can detect such situations and upsample minor classes? For sure it is related to https://github.com/mljar/mljar-supervised/issues/157 However, this issue requires rather a quick fix and #157 requires a larger treatment of unbalanced datasets.