[Unit Test Extension]: Implement "default setting unit tests"

[MalteKurz]

In the python package DoubleML, we do have unit tests for model defaults, see https://github.com/DoubleML/doubleml-for-py/blob/master/doubleml/tests/test_doubleml_model_defaults.py. The intention behind such "default setting unit tests" is twofold:

1. It should assert that the defaults are valid / meaningful, i.e., the code runs through successfully with default values for the input parameters.
2. The unit tests serve as a reminder to update the documentation of defaults in case a default value is being changed.

Such "default setting unit tests" could be done for the initialization of the model classes as well as for the most important methods.

Note: Such "default setting unit tests" would have been sensitive for bugs like #155 & #156