Add a coords/dims guide

[OriolAbril]

This guide should cover the following topics. 1) coords and dims definition in ArviZ (complementary to the conversion guides) 2) basic xarray selection and how this relates to the coords argument (also could be a good idea to cover var_names and filter_vars), 3) diving deeper into xarray. ArviZ itself uses a relatively small subset of xarray, even from a pure data storage prespective. In ArviZ each dimension has a coord with the same name by default, in xarray there can be many combinations, some of which I barely understand. One of the beauties of xarray is that as long as chain and draw dimensions are not removed or renamed, users are free to use any xarray feature and everything should still work. We should cover a bit this context info and then show some examples of bringing xarray features like multidim coordinates into ArviZ.

Thoughts on implementation

We have examples on several issues about mask, pointwise selection, multidim coords...

[mjhajharia]

basic xarray selection and how this relates to the coords argument (also could be a good idea to cover var_names and filter_vars),

I might take some time, but I wanna play around more with xarrays so I guess if I work on this, it'll probably give me a tangible roadmap to learn a bit and then write it down. I understand this isn't marked beginner friendly or something, so I get it if it's not an appropriate issue for me, but I'd like to work on it.

[OriolAbril]

Thanks! I actually forgot to link some issues in the original description with questions about slicing and coords usage that can be helpful.

• Example using multidimensional coordinates to label a dimension for a parameter? #1628 (for "section" 3)
• coords for pairs of variables. #1627 (for "section" 2 advanced)

[mjhajharia]

Thanks! I actually forgot to link some issues in the original description with questions about slicing and coords usage that can be helpful.

• Example using multidimensional coordinates to label a dimension for a parameter? #1628 (for "section" 3)
• coords for pairs of variables. #1627 (for "section" 2 advanced)

I'll have a look at these and get to it once I'm done making the inferencedata documentation PR. thanks!!