DOC: Add ellipticity example

[Lucia-Fonseca]

Description

This PR includes the validation plot for the 3D ellipticity model (Ryden04) in the examples page. It reproduces the distribution of axis ratio 𝑞_{am} for exponential galaxies in the SDSS DR1 as done by @ntessore here. Merging this PR closes #470.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

References

• Ryden, Barbara S., 2004, The Astrophysical Journal, Volume 601, Issue 1, pp. 214-220.

[Lucia-Fonseca]

To clarify:

• What does Ryden04 return exactly, the total ellipticity? See issue #475

[Lucia-Fonseca]

Review comments addressed.

[Lucia-Fonseca]

Some general comments:

• Calculating the variance from multiple simulations overcomplicates the code and distracts from the main purpose of the example. I would stick to just a single simulation as in our other examples.

Yes, I agree. However, how could you generate the error bars otherwise? Or you are suggesting we simply draw one sim and plot the histogram? How can we convince the SkyPy model is compatible with SDSS data?

• What is the source of the SDSS data? How was the file generated? Are we licensed to distribute it? @ntessore since I took this from your reproduce page. Do you have any idea?

[ntessore]

@ntessore since I took this from your reproduce page. Do you have any idea?

No, I queried the data from the SDSS servers because I didn't know.