Add option on NOAA datasource to aggregate by country and date

[ManuelAlvarezC]

Description

As the title indicates, now NOAA data source has a country_avg boolean option, with default to False, when True will return an average of the requested metrics by country and date.

Fixes #51

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
• [] Any dependent changes have been merged and published in downstream modules

[ManuelAlvarezC]

@DanielRobertNicoud, please wait to merge the PR #47, as the dataset validator has some issues fixed that I want to check in a skipped test in this PR.