The method of specifying arguments should be standardized.

[arafune]

ex: convert_to_kspace
coords (KspaceCoords, optional): Coordinate of k-space. Defaults to {}.
allow_chunks (bool): [description]. Defaults to False.
**kwargs: treated as coords.

The standard should be 'coords={"kp":np.linspace(-1,1,200)}'. kp=np.linspace(-1,1,200) is optional way, but to keep consistency and it should be unifid to coords=.

[arafune]

While **kwargs in convert_to_kspace can ratained, because it is somehow natural setting,
kwargs setting in fat_set (The width can also be specified by like "eV_wdith=0.1". ) is questionable.