Add a rough estimate for tutorial runtime

[fsimonis]

It would be nice to have a rough estimate on how long tutorials run, similar to "estimated reading time" on some websites. We could use the all serial setup to give these estimates.

While this is very hardware dependent, it can give the user a good feeling on what to expect.

For example elastic-tube-1d runs in approx 1 minute, heat-exchanger runs in approx 10 minutes, elastic-tube-3d in approx 1h (maybe).

[MakisH]

Good point, I thought we had such disclaimers, but apparently only on the heat exchanger: https://precice.org/tutorials-heat-exchanger.html

Other long-run tutorials are:

• Turek-hron FSI3
• 3D elastic tube
• any Nutils flow solvers

The default at the moment is "a couple of minutes". We could easily add alert boxes with disclaimers on the long-run tutorials.