What is dimension reduction anyway?

We've taken for granted that everyone knows what dimension reduction is. The scRNA-seq module should include more basic background info about dimension reduction in the scRNA-seq.

- @jashapiro wrote some of this out in the [scRNA-seq muddiest points notebook](https://github.com/AlexsLemonade/2020-july-training/pull/24).
- This should also possible include a quick note to distinguish the appearance of visual clustery points we get in dimension reduction from actual clustering analyses where clusters are actually defined.
- The scRNA-seq slides should probably include a visual to give participants an intuitive idea of what dimension reduction is before we get to it in the notebooks.

Start with the scRNA-seq module, its slides, and the scRNA-seq muddiest points notebook referenced in pull request #24. Review the existing dimension-reduction material, then add basic background, clarify how visual groupings differ from defined clusters, and include an intuitive slide visual. Done means the module and slides explain these concepts before participants use them in the notebooks.

Written by the indexing model from the issue text.