image-label: lost of original context

[jburel]

Dealing with multiple channels image with masks for example image_id = 6001247 (2 channels, 257 z) When the masks are converted to labels, it is collapsed to c = 1 in that case. At that point it is not possible to indicate which channel the mask should be associated to. The same holds true for any dimension that is "collapsed". Discussed with @will-moore @pwalczysko