Document new commondata format

[Zaharid]

The proposal for the new commondata format lives in the heads of some of us, and various partial implementations across pull requests, some of which are different. Ideally we would have some RFC style document discussing both high level ideas such as motivation (do we still want it to be "for experimentalists"?) and complete implementation details (i.e. something with as much information as the validobj stubs).

I believe this would make it easier to discuss changes and to actually port to the new format.

[Zaharid]

Examples of prior art include

• https://docs.nnpdf.science/data/exp-data-files.html?highlight=commondata
• https://docs.nnpdf.science/data/th-data-files.html#fk-table-compression
• https://docs.nnpdf.science/data/plotting_format.html?highlight=plotting+format

But we can always go better.

[scarlehoff]

I'll write a bit about the theory.

[cschwan]

In commit b5e26ea I added a preliminary document.

[Zaharid]

@cschwan Normally we prefer to proceed through pull request for these (and all) things. Also, please use RST instead of markdown for the docs (as per https://docs.nnpdf.science/contributing/sphinx-documentation.html?highlight=documentation). Markdown is only there still because it is a pain to remove and I can't be bothered.

[RoyStegeman]

Closed by #1708