Change RGB detection to be based on channel names

[aeisenbarth]

See #150

To give users more control over whether a 3/4-channel image is interpreted and visualized as RGB, channel names now need to be explicitly set to ["r", "g", "b"]. Therefore, to_spatial_image would have a new argument rgb=True to override the channel names (or auto-detect with rgb=None).

[aeisenbarth]

Since the condition {"r", "g", "b"} <= set(c_coords) does not require a specific order of c_coords, I should also reorder them to RGB, since that is what Napari will use. Does anyone know the corresponding xarray function (analog to transposing dimensions)?

[aeisenbarth]

Tests fail because multiscales blobs are not correctly set up with provided channel names. scverse/spatialdata#342 would need to be merged first.