Further refactoring and some improvements

[Bensk1]

This PR makes the following changes:

• Add frequency parameter to queries. Without specifically changing anything this does not have any effects because the frequency is set to 1 be default. This change is also debatable, we don't need it, but it was requested during reviews and is useful for further experiments with varying frequencies
• Replace hand-crafted requirements.txt with pip freeze-generated one that is more complete and contains versions
• Upgrade isort and adapt its call
• Unify MockConnector usage
• Fix a test error in test_algorithm.py that occurred when HypoPG was not initialized. (HypoPG is initialized per database, this test does not select a particular database which caused the error)
• Introduce assert for dexter being installed (otherwise the tests fail without proper error)
• Some formatting changes introduced by new version of black