Incorrect serialization of a cropping transform 

The attribute `crop_size`  needs to be renamed into `crop_to` to match the constructor. Otherwise, the serialization does not work well.

https://github.com/MIPT-Oulu/solt/blob/770e397884bcafe80a11723c229e275c1c1f8b5a/solt/transforms/_transforms.py#L715