Create beginnings of introductory vignette

Changes

Create basic infrastructure of an introductory vignette demoing how to create the reneeDataSet object and run deseq2.
Refactor to use S7 generics & methods.

(~Strikethrough~ any points that are not applicable.)

[x] This comment contains a description of changes with justifications, with any relevant issues linked.
[x] Write unit tests for any new features, bug fixes, or other code changes.
[x] Update the docs if there are any API changes (roxygen comments, vignettes, readme, etc.).
[x] Update NEWS.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/
[x] Run devtools::check() locally and fix all notes, warnings, and errors.