tests/test_advanced_image_processing.py: Changed percentage_pixels_below_max in the config variable in test_postprocess_data_2 to a floating point value.
tests/test_advanced_image_processing.py: Added a test for merging two neurons with advanced_image_processing.postprocess_data using 2D data.
tests/test_advanced_image_processing.py: Added a test for merging two neurons with advanced_image_processing.postprocess_data using 3D data.