draft of vignette

[svteichman]

This PR does the following:

• adds example data from issue Proper example & vignette #16, adds this to the package under name example_data and documents it
• adds vignette "intro_enviromtx"

@adw96 I did my best to describe this methodology based on the manuscript. However, I'd recommend that you especially double check my interpretations, and adjust these as needed! A few things to note:

• I didn't do any plotting or other exploratory analysis of the data. If you want me to add this let me know (I know Zinka provided some plots in issue Proper example & vignette #16, I can add something like that if you like)
• I added a citation but am not sure if this should be adjusted (since paper hasn't been finished yet)
• I added a technical appendix at the end describing the backend (raoBust and glm vs geelm, and how when geelm fails it calls glm, but if you want a robust score test in these settings you need to input cluster_corr_coef). However, feel free to remove this if you think it is too much for an intro vignette!