Add coverage badge and workflow

[cianc]

Description

Add coverage badge and workflow

Related Issue

Please link to the issue this PR resolves: #974

Motivation and Context

Adding a coverage badge.

How Has This Been Tested?

Ran and checked coverage locally. Checked that https://codecov.io/gh/mlco2/codecarbon/graph/badge.svg (but is "unknown as expected).

Screenshots (if appropriate):

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[benoit-cty]

Thanks !