LaTeX: use \setparstyle in sbs panels

[Alex-Jordan]

\setparstyle makes sure that certain environments use the tex document's global values for \parskip and \parindent. It is not presently used in an sbs panel. You would never notice this until you have a sidebyside/stack with more than one p. You can see it here:

https://pretextbook.org/examples/sample-article/derivatives.pdf#page=144

But it is not the best example because those panels are so narrow. In the second column, you can see where the second and third p start only because you can see the previous paragraph ending shy of the right edge. I think you would want the second and third p to be indented. (And if using a nonzero \parskip you would want to see that too.)

So this change is so that sbs panels use \setparstyle.

[Alex-Jordan]

If this is considered a good change, I propose making a similar change for dl. The same issues arise there if an item has multiple p. I could add that to this commit, but I wanted to check your opinion on one item at a time.

[rbeezer]

Yes, that looks better. Can you work up a PR for dl?

[rbeezer]

Merged. Will updare website samples soon, but not this minute. Thanks! We'll get this all looking perfect eventually!