Stitching multiple channels

Hi @camilolaiton ,

I am having difficulties in stitching multiple channels. I tried to modify the `processing_manifest.json`, but the stitching code does not accept multiple stitching channels as a list.

Does the pipeline only stitch with one channel, then apply same transformation across all channels at the fusion step? If that is the case, which channel do you choose to generate the stitching transformation and why?

Thanks!

Best,
Yang