Create RNA-seq example of differential expression with several groups

[cansavvy]

Now that we have a microarray DE example for more than 2 groups, I think it makes sense to have one for RNA-seq since people may want to know what changes are needed for the DESeq2 differential expression steps for multiple groups.

Here's the section of the DESeq2 vignette that describes how to do contrasts and that we should use when creating this example: https://bioconductor.org/packages/devel/bioc/vignettes/DESeq2/inst/doc/DESeq2.html#contrasts

The biggest step for this example will probably be finding an appropriate dataset with enough metadata associated with it and a big enough sample size (20 or more would be nice).

We may want to also think about what other aspects of a DE analysis we may want to illustrate here that we haven't shown yet. (e.g. Identifying an outlier?) But a lot of this will depend on what the example dataset we use naturally has going on with it.