Improve QA workflow

[ManuelAlvarezC]

In order to improve the github workflow the next improvements should be done:

• Add codecov on the CI build.
• Create a test that check on data_source module and run it on new PRs.
• Create a Mantainer Guide. It should contain:
  1. Mantaining issue section.
  2. Making PR reviews.
  3. General QA.
• Create a couple of functions that generate templates for datapackage.json and audit.md using the output of a data source.

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