WB_WDI source improvement

[nicolas-graves]

Add WDI dataflow/codelist/datastructure.

However, it uses a lot of unconventional quirks, which I would be happy to correct.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[nicolas-graves]

The tests are failing, but I'm not sure it has something to do with the PR itself.

[nicolas-graves]

This is actually not working that great because it relies on the assumption that dataflow is called without arguments, which is not true in the general case.

[khaeru]

I still will need to find time to look more closely at what this is doing.

In the meanwhile, one question for context: is this the same as or similar to this workaround that I have downstream?

In essence, there I found that the WB_WDI source does not really implement the SDMX-REST API per the standard. I responded by directly accessing a URL from their documentation. Is this PR similar in trying to handle a certain documented/known non-standard API behaviour? If so it will be important to have (or reference) a clear statement of what that behaviour is.

[nicolas-graves]

@khaeru It seems indeed to be the same thing for codelists as the link you pointed at. I think the dataflow implementation on my side is most probably erroneous and should be discarded (since there are dataflow calls in higher level commands). I'll check to verify that codelists and datastructures are not used in other functions.