[grafml] Adding tests

[MarcelGraf0710]

As requested in the exercise, I completed the following tasks:

1.) Added assertion statements to the functions initialize_domain and initialize_physical_parameters
2.) Wrote unit tests (in my case using pytest)
3.) Wrote integration tests (again using pytest)
4.) Checked the test coverage and generated a PDF file that is also included
5.) Created a tox.toml as well as the corresponding requirements.txt