Introduce coverage report generation and generate GitLab coverage badge via GitLab feature

Duniterpy display a badge in Gitlab for tests coverage, but it is broken.

This badge depends on an old cloud service workflow involving github, coveralls.io and maybe via travis. But the workflow is not working anymore.

We can repair the broken badge without any cloud platform, by following this method :

Edited Jun 08, 2021 by Moul

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