On groupby/reshaping/pivot tables

[nickeubank]

I'd like to advocate for a few changes in these materials:

• In the groupby readings, adding discussion of .transform() and as_index=False. If it's helpful, because I've historically used other people's books on these topics, you'll find some of my own preferred instructional materials buried in this exercise
• I think we should have a standalone reshaping exercise that includes discussion of the properties of a "tidy data" dataset format looks like (as motivation). You can also find how I introduce the zoo of methods for reshaping here.
• Finally, I'd add a new section for "pivot tables" (it's a term people are so used to from excel that even if it's redundant with what I think belongs in reshaping when it follows groupby, I think we should have a break-out reading on it). Also while I think it's ok to introduce pivot_tables, I would emphasizes that groupby + reshape do the same thing.

[kylebradbury]

@nickeubank - I've made each of the additions above in the commit referenced in this issue. I tried to keep the reshaping reading short to avoid getting too deep into the weeds. I believe I touched on everything you mentioned here, but let me know if you see anything missed.