fix: Add namespace member variable to all ml classes

Infering the namespace from the filepath is suboptimal, because users may not use the project structure properly. Moreover this feature is not documented. Therefore an additional constructor argument has been added to propagate the namespace information.

Also there are tests to ensure, every class provided by DT_FACTORY can be instantitated. Furthermore a filter has beeen added that prevents classes that don't belong to the ml hierarchy to be inserted into DT_FACTORY.