ENH: adding euclid clusters tutorial

[bsipocz]

This is a draft PR for now as the notebook runs way too long as is (2+hr locally)

TODO (that came up during making this PR, we may have other things left from the initial review):

• fix figure captions
• add metadata footer
• make sure it runs and runs well within the confines of CI
• cut up code cells, they are way too long and with little narrative in between

Extra things that are definitely not merging blockers, but what would make an improvement:

• paper parsing for the table could be done with astropy, that may eliminate the need of introducing a new dependency (e.g. currently the notebook is failing due to the missing lxml library)
• consider not using seaborn, as it is now a new dependency. If we were to use it in other notebooks, too then that's fine, but if it will be only this one, then I think we should reconsider to stay with mpl.

Resolves #282

Closes #283

[jkrick]

@bsipocz do you want me to look at how to shorten the runtime of this notebook? and also do you want me to look into adding narrative text? or do you already have a plan?

[bsipocz]

I would appreciate any help, thanks for offering.

[bsipocz]

@jkrick - you should be able to directly push to this branch. Based on what you showed today I think will know how to do that, but please ping me if you need any help.

[bsipocz]

Runtime actually doesn't look as bad on CI, I'm not sure why I had more issues locally. That being said if there is a reasonable way, maybe we could still cut it back to ~half?

[jkrick]

I sped up runtime with some low hanging fruit - by parallelizing the download and downsampling the images for display. Let me know if this isn't enough.

I'd prefer to do a major text wrangling in a separate PR, so I will make a separate issue for that, and please don't wait for it in this PR.

[bsipocz]

We're hitting circleCI memory limits with the new notebook, so I add it to the skip execution list for now.

[bsipocz]

I'm going ahead and merge this PR. Jessica said above that she was planning on working on a follow-up PR and the remaining items are not blockers either.