Summer 2021 to do list

[Sean Fitzpatrick]

I'm going through the issues on GitHub and on here, trying to sort out what I can reasonably try to tackle this summer (or talk someone else into tackling).

- indexing for Riemann sums is dealt with now.

- there are a few small edits/enhancements: examples with manipulation of power series, error estimate for integral test, multivariable implicit differentiation. I think Grady has also added something on GitHub

- spherical coordinates and concavity

- PDF styling

For the last, I'm fairly happy with where things are at, but Greg should review. (See, e.g. https://opentext.uleth.ca/PDF/apex-standard.pdf)