add an image to represent the Seurat object

[mistrm82]

this will be helpful in the first lesson to understand how the slots/layers work. Also, we can reference it in the cheatsheet

[mistrm82]

add homework question/exercise to explore the Seurat object

[mistrm82]

I think @Gammerdinger and @nsohail19 were working on/discussing this image at some point? If we can get this done for the end of the month that would be great (including a few exercise questions)