IMF/WEO : each major release as a new dataset

As a user, I want to access the different major releases of WEO from the IMF. There are twice a year (April and October). On the provider website, I can retrieve these different releases. For instance, April 2020 https://www.imf.org/external/pubs/ft/weo/2020/01/weodata/download.aspx and October 2019 https://www.imf.org/external/pubs/ft/weo/2020/01/weodata/download.aspx. To be consistent with the provider website, we must create on DBnomics a specific category, WEO, and consider these different releases as a new dataset.

Remarks :

• This option will avoid issues reported by different users about missing series from one release to another (#687 (closed)).
• This is consistent with what we've done with EO from OECD
• Each major release also contains revisions. This option will allow to deal more efficiently with this.

The latest revision can be accessed via the "virtual" dataset code {dataset_code}:latest.