[TEST] testing the software

- **Difficulty (1-10):**
it will take some time
- **TestFiles:**
all
- **Which code gets tested:**
a few parts
- **What should be done:**
we have to add all way more unittests.
To do that we have to ensure that every function has one task #18
- [x] each function does only do one task

Testing:
1. add am unittest for each function to test:
- its input and output types (signature)
- its behaviour
2. add an unittest for each class, to test out the API of the class over the lifetime of the object