Adding Object-Oriented tutorial - #410

I'm going to need a bit of help with this PR. The section headers yield warnings and don't render, saying:

 WARNING: Title level inconsistent:

Thanks @bmorris3! I'll let others take a closer look at the content, but a few comments:

• You can ignore the title-level inconsistent warning
• Could you rebase on master to fix the circleci failure
• It is probably worth noting in the tutorial intro that there exist some existing implementations of light curve or time series classes (I'm thinking of http://docs.astropy.org/en/latest/timeseries/index.html and https://github.com/KeplerGO/lightkurve), so this is primarily meant for educational purposes

Colab notebook here: https://colab.research.google.com/drive/1A02gYb6aOOcXUnb0MlUKFoNX9aEtDJ6N

I think the issue that both @adrn and @bsipocz brought up about the time series class is an important one. Maybe y'all can discuss? Or we can discuss on the learn telecon this morning.

The content of the tutorial itself looks pretty good. It would be useful to point out that in this specific case there are a couple of potential packages that users should use instead of making their own class: astropy time series and lightkurve.

I think it is actually good to have as an example something that has a better/more complete implementation elsewhere.

👋 We have changed the build infrastructure and layout of files in the repo, so I am going to rebase this pull request and force push to your branch. If you have any local changes that haven't been pushed, please push them by the end of this week (October 1). After I have rebased to bring the new build machinery in, please let us know if you would like to continue working on this tutorial or if you would rather someone else take it over. Thanks!

Hi @bmorris3!

I realise it has now been a number of years since you submitted this, but it looks basically complete. The only feedback that I see that hasn't been implemented is pointing the user to the existing light curve packages. I am happy to add this, if you are willing.

I apologise for how long we've let this sit. Let me know what you think.

Sure @ceb8. Since I made this PR, I put a better version of this notebook in astropy-workshop that I could push to this branch instead if you like.

@bmorris3 That one looks great! Yes please push it here. Thanks!