RGB tiff file conversion should create RGB datasets

[markbader]

Context

• Affected library: e.g. webknossos
• Uploading an RGB dataset as tiff (RGB stored in s channel) results in a 4D grayscale image

Expected Behavior

• When converting a tiff file with sxyz axes, with s representing RGB values, the resulting dataset should be an 3D RGB dataset instead of a grayscale 4D dataset

Steps to Reproduce the bug

• Reach out to me for an example dataset