Implement Comprehensive Tests for the Plugin

Note that the plugin was developed on a tight time schedule and is missing tests.

Add:

• Unit tests - E.g. If hole extraction works as intended, if the renaming of the output layers work as intended, ...
• Integration tests - If the individual helper scripts are working by themselves (step1's and step2), if the submodels handle TLM input layers with the "objektart" attribute being int AND/OR str, if the scripts still produce the correct output after setting the outputs to "optional", ...
• System tests - A ratio between the produced settlement and the settlement produced by the old methods of the Federal Office of Spatial Development to see how accurately these models calculate the settlement, ...
• Performance tests - after optimization/multithreading is implemented, what are the time requirements to calculate the settlement for e.g. entire Switzerland (or only a small region of interest), ...

Have a look at this conversation with many useful suggestions and links: https://gis.stackexchange.com/questions/382998/unit-testing-for-qgis-3-plugins