Demonstrate (indirect) support for Seurat

[kevinrue]

Add a section "Compatibility" that documents alternative inputs to SummarizedExperiment objects that can be fed to iSEE with a bit of extra work.

Start with Seurat objects, with an explanation of why we haven't written our own conversion function, instead leaving users to choose from those already existing out there.

Relevant citations:

• https://academic.oup.com/nargab/article/2/3/lqaa052/5877814