create test methods to verify a custom class implementation
Create a test suite that can be used for default testing during CI as well as for an user of MLAir.
By using this test suite, a custom class e.g. a new data handler or model module can be tested, if it fulfils all requirements.
Tests to implement:
- init (maybe split init and _run, and call always init and run of a stage -> refac)
- can run?