[Feature] Handling RGB images as input

[IgorTatarnikov]

Currently the package cannot differentiate between a 3D image or an RGB image leading to undefined behaviour.

We either enforce that the input data is greyscale or handle the RGB images internally.

I'm leaning towards handling RGB images internally. We'll have to convert to greyscale before passing to elastix but all operations outside of elastix should recognise 2D RGB (and 3D RGB) images and perform operations accordingly.

[IgorTatarnikov]

This should also standardise how the dimensionality of data stored in napari layers is accessed. Accessing the ndim property of napari.Layer accounts for 2D RGB vs 3D images, while accessing the ndim property of napari.Layer.data does not.