Examples with aggregated data from multiple experiments

> I guess I haven't been thinking about the aggregate cases since none of our examples or planned future examples use the aggregate option.

_Originally posted by @cansavvy in https://github.com/AlexsLemonade/refinebio-examples/pull/232#discussion_r504162569_

One of the major features of refine.bio is the ability to aggregate data from multiple experiments into a single downloadable expression matrix. We should have examples that highlight this! Of course, it is something that can be quite fraught with caveats, but that is all the more reason to have a use case that presents reasonable ways handle such data. This might include some batch correction, or maybe just highlighting clustering by platform that might occur.

This issue should be split into specific ideas, but I thought a placeholder was worth filing!

Start by breaking the placeholder into specific example ideas for aggregated data from multiple experiments. Review the proposed expression-matrix use cases, including batch correction and clustering by platform, and define what each example should demonstrate and how caveats will be handled. The issue names no files or tests; done means selecting and specifying concrete examples.

Written by the indexing model from the issue text.