Proofreading and editing

[mitchkeller]

@nelong This is a WIP branch/pull request so that you can see what I've got going with editing. I'll add comments here as I push edits. Not ready to merge until there's an update saying so.

• Files have been renamed to eliminate chapter/section numbers as part of filenames. This is also being done systematically to xml:ids as I edit.
• In addition to systematically cleaning up xml:ids and xref syntax as I go through, I am trying to add a description element to each image element so that someone using a screenreader will find the image more accessible.
• 12.1 is done. I hacked together something to deal with the one remaining figure that was produced in Mathematica. It's not perfect, but it conveys what we want.
• 12.2 is done. Because it seems that you can't animate a parametric plot in SageMath, I threw a CalcPlot3D applet in here after the activity about oriented curves. It seems to look good and is very simple to use. If you have an easy alternative in SageMath, I'd be happy to swap this out.

[mitchkeller]

I added an aside to the Idea of Line Integral section about other notation for line integrals. Give that a long sometime, @nelong, to make sure I've not lied and that we say what is important to say!

[mitchkeller]

@nelong I have finished the edits to ParamLineIntegrals, other than adding the exercise on changing notations that we discussed on 23 June 2020. The only significant change I made was in Activity 3, where I changed the curve in part (a) and thus the points in part (b). I thought it was more pedagogically sound to not have the line integrals in this activity all work out to 0. I also added a few parts that explicitly ask the students to make connections between the various activities and parts thereof to scaffold what is going to come with FTCLI.