Unit tests for sound fields

[hagenw]

One way to create meaningful unit tests for our driving function might be to create sound field plots save them as image and do a diff between the images.
This is also used by matplotlib, compare matplotlib/matplotlib#10613.

[fs446]

Images might have been compromised by data->pixel interpolation and color quantization. Although this might cause insignificant contribution I'd prefer store Python's/Matab's original numeric data rather. In order to evaluate the full signal chain we'd need numeric data of the driving functions, secondary source locations, acoustic transfer functions and final sound field and probably a lot of other stuff I forgot right now. From my point of view, some of these are even inconvenient to check with images.