Removes missing values from input dataset.

The following modes are supported:

- Mean   - replaces missings with the mean of fit column
- Median - replaces missings with approximate median of fit column
- Custom - replaces missings with custom value specified by user

For mean and median modes, only numeric column types are supported,
specifically:

- int
- long
- float
- double

For custom mode, the types above are supported and additionally:

- str
- bool
