Fix cds datasource

[ManuelAlvarezC]

Description

Changes wrong values on aggregate columns for rows with state value but no value on state, now region column. Add tests for dataset validator. Fix open issue with the validator.

Fixes #41 Fixes #44

Checklist:

• [x] I have read the CONTRIBUTING GUIDE and made sure this Pull Request is compliant with it.
• [x] I have read the DATA MODEL and DATA SOURCES and made sure this Pull Request is compliant with it.
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules

[DanielRobertNicoud]

In main.py the argument of the function should be country="Italy". As it is currently, it returns an empty dataset.

IMO, the connector should optionally take a list of countries, not a single country.

The argument should be ISO-2 (to be fixed later on).