stub out a demonstration of how integrative analysis could work

[mccalluc]

Yesterday morning I had thought that the new thing would just be a composition of datasets... but if it actually embeds a viewconf, then it's reasonable for it to be handled here: It won't take much code, and this will let us:

• set the has_visualization facet at index time. (I think that's the name?)
• catch some errors at index time?
• use vis_preview.py consistently. (I think the pipeline developers have used it to preview the dataset before publication.)

FWIW: I'd go for a longer vitessce hint string than just uia: Bytes are cheap! Comprehension is expensive!

Filing as draft: Prereqs need to be in place before this can be used.