Feature/add coveralls test coverage

[SabineHaas]

Address #257

Changes proposed in this pull request:

• Test coverage is now automatically checked with github actions and coveralls

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

• Update the CHANGELOG.md
• ❌ 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
• 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.

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

[SabineHaas]

Locally I can generate a coverage report, which is printed in the terminal. However, on github actions it errors out with

Run coverage report -m
No data to report.
Error: Process completed with exit code 1.

see https://github.com/greco-project/pvcompare/pull/283/checks?check_run_id=2325216239

Discussion also in MVS: rl-institut/multi-vector-simulator#669

[SabineHaas]

Open todo after merge of this PR (#283):

• adapt badge in README.rst

[MaGering]

Looks good! Coverage report runs locally for me too. Thank you @SabineHaas