Spectroscopic Data Reduction: trace, extract, calibrate & data

[keflavich]

Tutorial 3 following #508, #510

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[adrn]

I fixed the issues with the color-excess tutorial (causing your builds to fail), so if you rebase this and your other PRs (sorry...) the CI should start working again. Sorry for the headache!

[eblur]

Overall looks good. My only ask is to make the notebook titles in this series more clear (Part 1, 2, 3 etc with sub-titles defining the end goal of each notebook).

[keflavich]

I've addressed @eblur's comments and applied some other changes that were needed (header, import location, etc)