various edits to warmups and cooldowns

[jkostiuk]

addresses comments in #374 for easier cross-referencing for analogies.

I opted to splice in a remark at the beginning of each warmup or cooldown in which the analogy is revisited. The remark links to the original warmup but also reminds the reader right there what the terms mean.

[jkostiuk]

I added the intro and commited/synced the changes. Is there an official "ready for review" button I should click or some other part of the process?

[StevenClontz]