Reorganize the pytest-mpl reference images

[janssenhenning]

At the moment updating the pytest-mpl baseline images is a real pain, since we have multiple different directories with baseline images with similar names and pytest-mpl can only generate baseline images in one flat folder, where images with the same name simply overwrite each other

The easiest solution would be to move the test functions outside of classes and put the plot function name in the test function name. Then we can define one baseline directory and use the standard @pytest.mark.mpl_image_compare without options to generate the images