Add actions for unittests

[MGlauer]

This PR introduces automated tests that run on every pull request. It runs python -m unittest on multiple python versions.

[MGlauer]

Tests fail in general, this is not a fault in this branch (#42)

[sfluegel05]

I made the following changes:

• Merging the unit tests from PreProcessing unit tests #48 into this branch
• changing the test command to python -m unittest discover -s tests/unit (this ignores the integration tests which have been the cause of Unittests are failing #42)
• removing a deprecated typehint
• removing tests for pypy

With that, all checks are passing.