Add documentation on restarting CalculiX simulations

[MakisH]

This PR shapes this Medium article by @moon1144 (written for a much older state of preCICE) into a section of the CalculiX adapter documentation.

I did not have success restarting yet (my flow simulation crashes after a few iterations in the first time window after restarting), but I think this is a first step long overdue.

Why now:

• I recently found out about @moon1144's article (I think suggested by @IshaanDesai) and did not want to let it slip through
• this year's workshop is approaching, and last year I talked a bit about this with @MatthiasFreimuth
• @vidulejs is currently debugging some simulations using restarting, so he might have some additional insight

All kind of feedback welcome right now. This is something that many users are asking for often.

[MakisH]

Additional notes on my failing case:

• I am using the perpendicular-flap tutorial
• I am trying both with and without FSI { restartFromDeformed true; } in the fluid-openfoam/system/preciceDict.
• I am running until and restarting from 0.5s
• I am not cleaning up before running again, just setting startFrom 0.5 in the controlDict and using the modified input file for CalculiX.
• I am setting just <max-time value="0.5" /> in the preCICE config. The max-time-windows stated in the article should not be needed.

[vidulejs]

The perpendicular-flap case doesn't use NLGEOM I believe

[MakisH]

Thanks for the observation! This shouldn't matter though, right?