Better periodic boundary examples

[pjaap]

This implements a better periodic boundary example to demonstrate the feature.

It is a 3D linear elastic plate glued in the central part of the bottom with an upwards pulling force.
Left and right boundary faces are combined periodically.

However, currently, it shows a strange error, that needs investigation.

The periodic boundary faces are pulled down...

[pjaap]

3D and 2D examples look great now!

[pjaap]

Thanks for the better grid. I'll try to eliminate the test failure and clean up the examples. Then, we'll just merge.
Should we keep both examples?

[chmerdon]

Ok. Yes, we can keep both.

[chmerdon]

the descriptive comment blocks at the beginning of the examples need an update, though

[pjaap]

Yeah, that's part of the cleanup

[pjaap]

Ok, please take a look. I'll squash this into one commit if the test runs through (and included the two examples!!)

[chmerdon]

Looks good to me now! Are we also happy with the way the 3D body slightly bends downwards in the middle of the periodic boundary?

[pjaap]

Thanks! Forgot to add the two examples in the test.

Commits are squashed. Waiting for CI to succeed.

[pjaap]

Looks good to me now! Are we also happy with the way the 3D body slightly bends downwards in the middle of the periodic boundary?

I tested it with the domain glued at the end without any periodic boundary and it looks the same. So the bend is a material property.

[pjaap]

Please let us merge if CI is happy. The max value seems to be a little bit different on different systems. I lowered the test precision.