Open result paper and DESC note draft for the challenge

[EiffL]

This PR adds a paper and DESC note draft for the tomo challenge. After concertation with @aimalz we think the following publication plan would be the best outcome:

• A short DESC note that describes only the challenge, to be freely used as the reference for anyone wanting to use the dataset.
• A paper on the challenge results.

I do think this project deserves a dedicated paper, even if challenge results paper can often be difficult to frame. One way to limit the scope of this result paper and make it readable would be to only include a small paragraph on each method in the main text, encouraging participants to publish their own method paper separately which we can cite, or failing that, an appendix section.
And otherwise, I think the comparison of strategies and insight we gained from the challenge would I think make for an interesting paper (similar to https://arxiv.org/abs/1901.06495 )

I have included a very preliminary list of authors based on participants to the challenge and people I knew where DESC members. Not my intention to exclude non DESC members, I'll just need to reach out to them individually so that we can setup an external collaborator request.