Perpendicular flap volume example and shell example

[Crazy-Rich-Meghan]

Added two variants for the perpendicular flap example.

1. Solid participant uses 3D elasticity geometry
2. Solid participant uses 2D shell represents 3D elasticity

[Copilot]

Pull Request Overview

Adds functionality to extrude 2D spline surfaces into 3D volumes and provides two new “perpendicular flap” example applications—one using full 3D elasticity and one using a shell-based approach.

• Introduce surfaceToVolume utility and a wrapper overload in gsPreCICEUtils.h.
• Add a PreCICE XML config for the shell-based 3D example.
• Add two example executables: one for vertex-based 3D elasticity and one for shell-based volume extrusion.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
gsPreCICEUtils.h	New surfaceToVolume utility and toVolume wrapper
examples/perpendicular-flap/precice-config-shell-3d.xml	Configuration for shell-based coupling example
examples/perpendicular-flap-vertex-gismo-volume.cpp	Example using 3D elasticity geometry
examples/perpendicular-flap-shell-gismo.cpp	Example using 2D shell representation for 3D volume

Comments suppressed due to low confidence (1)

gsPreCICEUtils.h:500

• [nitpick] The @see tag references toVolume itself; it should reference surfaceToVolume where the core implementation lives.

 * @see toVolume(const gsTensorBSpline<2, T> &, const gsFunction<T> &)

[hverhelst]

I made some small changes to the PR. Now it looks good to me.