Add codecov test coverage

[SabineHaas]

Fix #257

Changes proposed in this pull request:

• Add codecov test coverage to github actions workflow

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code
• Update the CHANGELOG.md (let's start this after the first release)
• Apply black (black . --exclude docs/)
• Check if full simulation tests pass locally (EXECUTE_TESTS_ON=master pytest)

Please mark above checkboxes as following:

• Open
• Done

❌ Check not applicable to this PR

_{For more information on how to contribute check the CONTRIBUTING.md.}

[SabineHaas]

I close this PR, issue was solved with #283 and #289