Closes #3378: Update chpl tutorial (#3507)

* Closes #3378: Update chpl tutorial

This PR (closes #3378) is a rather substational update of the chpl tutorial since it's been quite a while. I added sections, changed explanations, and attempted to better motivate concepts instead of just introducing. I think this is a big improvement from what we had, but I did lose steam towards the end so I won't be surprised if there are lots of mistakes.

Given there's so many changes and moving things around, I think it probably easier to just look at the md file on my fork than the diff

* more explanation for aggregation

* Updates in response to PR feedback

* moved a paragraph

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Co-authored-by: Tess Hayes <stress-tess@users.noreply.github.com>