Finalize test coverage

[SabineHaas]

Fix #257

Changes proposed in this pull request:

• Move coverage badge of coveralls.io from deprecated to valid section in README.rst

The following steps were realized, as well (required):

• [x] Update the CHANGELOG.md
• :x: Check if full simulation tests pass locally (EXECUTE_TESTS_ON=master pytest)

Also the following steps were realized (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

Please mark above checkboxes as following:

• [ ] Open
• [x] Done

In case of an error due to linting, run black . --exclude docs/ and push your changes. Note that in case you do not fix a whole issue you should start your PR with Address #xyz.

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