Document script plot-all-displacements

[MakisH]

Very use-case-specific, outdated, and trivial to reproduce from plot-displacement.sh

[BenjaminRodenberg]

The purpose of this script is to quickly be able to reproduce this plot:

https://github.com/precice/tutorials/blob/master/perpendicular-flap/images/tutorials-perpendicular-flap-displacement-all-watchpoints.png

As far as I remember this is to some degree a regression test we do more or less regularly. We should at least somehow document how this plot is created.

[BenjaminRodenberg]

I think we should not delete this file and keep everything as it is for the distribution release:

1. Comparing runs {SU2, OpenFOAM, nutils} X {FEniCS, CalculiX, SU2} is valuable. E.g. #250
2. Running all 9 cases and storing them under unique names requires manual effort. I don't think we can easily improve this situation. Giving a name to the watchpoint file that includes the names of both participants (i.e. openfoam-calculix.log) breaks the black-box. The participant that is creating the watchpoint file does not know which participant sits on the other side.
3. plot-displacements.sh only allows to plot a single curve, not multiple. Extending this to support plotting of several is not trivial in this setting.

Possible todo: Documentation how to do the runs, store the results, and create the plot (manual effort mentioned under 2) could certainly be improved, but this is not really something critical for the distribution release.

[uekerman]

I would keep the script and add 2 sentences of documentation to the case README.

[MakisH]

1. Comparing runs `{SU2, OpenFOAM, nutils} X {FEniCS, CalculiX, SU2}` is valuable. E.g. [Change C3D8 elements to C3D8I elements in perpendicular-flap solid-calculix #250](https://github.com/precice/tutorials/pull/250)

The comparison is valuable, the script as-it-is is mostly confusing. I restored the script and added documentation.

2. Running all 9 cases and storing them under unique names requires manual effort. I don't think we can easily improve this situation. Giving a name to the watchpoint file that includes the names of both participants (i.e. `openfoam-calculix.log`) breaks the black-box. The participant that is creating the watchpoint file does not know which participant sits on the other side.

Automation could help, but that would be an overkill even for the system tests, as we need a system with all the solvers installed. We have that already in the VM, but even that would probably take hours to run. Reminder: we now have a cross-product of 3x7 (=21) solvers, with at least Nutils taking quite some time to run. Such automation would need to take fault tolerance into account.

3. `plot-displacements.sh` only allows to plot a single curve, not multiple. Extending this to support plotting of several is not trivial in this setting.

It is. Compare:

gnuplot -p << EOF                                                               
	set grid                                                                        
	set title 'x-displacement of the flap tip'                                        
	set xlabel 'time [s]'                                                           
	set ylabel 'x-displacement [m]'
+	set term pngcairo enhanced size 900,654
+	set output "images/tutorials-perpendicular-flap-displacement-all-watchpoints.png"
- 	plot "$1/precice-Solid-watchpoint-Flap-Tip.log" using 1:4 with lines title "$1"
+       plot "watchpoints/openfoam-calculix.log" using 1:4 with lines title "OpenFOAM-CalculiX", \
	     "watchpoints/openfoam-dealii.log" using 1:4 with lines title "OpenFOAM-deal.II", \
+            ...
EOF

The difference is in the hard-coding of the filenames (just more than one arguments). The two set should actually also have been used in the simple plot-displacement.sh.

Possible todo: Documentation how to do the runs, store the results, and create the plot (manual effort mentioned under 2) could certainly be improved, but this is not really something critical for the distribution release.

I would keep the script and add 2 sentences of documentation to the case README.

I added more documentation.

We now essentially need to do the manual run of everything and update the picture. I will try to do that next week in the VM, in a separate PR.

[BenjaminRodenberg]

Minor comment. Rest looks ok.