Currently, the NBeats blocks are "fused" with the KAN design, it is simply an if/else in most cases, based on the boolean parameter use_kan.

This is not good practice, so we should split the NBeats layers, NBEATSBlock, NBEATSSeasonalBlock, and NBEATSTrendBlock into two - an NBEATSBlock and an NBEATSBlockKAN, etc.

The kan_params should become explicit kwargs of __init__ rather than being passed as a dict.

Current use of use_kan=False should map to NBEATSBlock, and use of use_kan=True to a new NBEATSBlockKAN, with parameters passed explicitly.