Improve test coverage

[PhilippSchuette]

At the moment the test coverage for the two modules settings and plugins is quite low in comparison with the rest. Additional tests should be implemented to remedy this situation.

[PhilippSchuette]

In addition, setting verbose=False for all root finder tests affects coverage of rootfinder.py and parallel_finder.py in quite a bad way. This should be fixed by mocking a FinderProgressBar. To make this easier the concrete instantiation within the RootFinder should be replaced by resolving the bar from the ServiceLocator.

[PhilippSchuette]

Argument estimation with order>0 lacks testing as well right now!