Add Covid related series

[ardimirzaei]

Summary

Added downloader for COVID related data from John Hopkins GitHub. Mean aggregation over the week and take diff of the dates.

Related Issues

52

Checklist

• [x] The change is tested and works locally.
• [x] All related issues are referenced.
• [ ] Blog post included in PR if required e.g. new feature.

[ardimirzaei]