Add ChannelIndex to MagLocator, adapt Zarr Import for Multichannel datasets

[fm3]

Remote Zarr datasets can have n channels. wk can only handle single channels per layer (or uint24 rgb layers, which is a special case). To import multi-channel datasets in wk, the MagLocator could store the channelIndex, and data loading could then use the selected channel. Multiple layers could then each load one of the source channels.