Nutils cases are extremely slower than the rest

As stated in the Nutils Book, Nutils is not (yet) the fastest tool in its class. I experience that in our perpendicular-flap tutorial, where especially our fluid-nutils case is taking orders of magnitude longer than the rest of the solvers (half a day instead of a couple of minutes).

Following the official suggestions, I tried something like

OMP_NUM_THREADS=1 NUTILS_NPROCS=4 python myscript.py

without a directly visible impact (did not measure). I also commented-out the VTK output at the end of each coupling time window.

However, I think that we should first have a closer look and check if we are doing any extremely unnecessary work (e.g., too tight convergence measures, too much output, etc). The mesh already seems to be very coarse.

@gertjanvanzwieten @uekerman do you maybe have any quick ideas?

If we could drop the simulation time to a couple of minutes (or anything less than an hour), we could also integrate this in our system tests.

Start with perpendicular-flap/fluid-nutils/fluid.py and compare its runtime with the other solver cases. Measure the effects of NUTILS_NPROCS, OMP_NUM_THREADS, convergence settings, and VTK output, then confirm that the case completes in under an hour before considering it for system tests.