It is unclear why the results changed. However the issue of only the drag error
increasing for higher refinement levels still exists.

Geometry and parameter choice was not quite suited to reproducing [SchaeferTurek96].

Using the pdfcairo terminal for generating fluid flow plots in gnuplot
using raw paraview-generated CSV exports generates high quality results.
Sadly the resulting files are both unoptimized and quite big which leads
to both large and unperformant documents (i.e. some PDF readers even crash)

As an alternative this new gnuplot structure generates the fluid plot
itself using the pngcairo terminal and includes the resulting
high-resolution PNG image into a normal pdfcairo plot. This works out
quite well.

As a side benefit the plot scripts are now much easier to maintain.

i.e. experimental evidence for naming border conditions inside of
     coupling areas root of the problem.

Leads to big files but is of much higher quality than anything
print-ready that I can convince ParaView to generate.