Principle stretch computation hotfix

[hverhelst]

IMPROVE:

• mark functions for depreciation,
• improve principle stretch computation

[Copilot]

Pull Request Overview

This PR marks legacy stretch and stress methods as deprecated and enhances the implementation of the principle stretch computation.

• Deprecates old pstretch_into, pstretchDir_into, pstress_into, pstressDir_into, parameters_into, and transform_into in the base class.
• Introduces new eval3D_pstretch interface and specialized implementations for compressible and incompressible cases.
• Updates include paths and removes an unused variable.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/gsThinShellUtils.h	Updated include path for gsExpressions
src/gsMaterialMatrixNonlinear.hpp	Added eval3D_pstretch methods and left a stray debug call to remove
src/gsMaterialMatrixNonlinear.h	Declared eval3D_pstretch override and its template overloads
src/gsMaterialMatrixBaseDim.hpp	Removed unused variable in deformation method
src/gsMaterialMatrixBase.h	Marked several legacy methods as GISMO_DEPRECATED

Comments suppressed due to low confidence (2)

src/gsMaterialMatrixNonlinear.hpp:568

• New eval3D_pstretch method introduces significant logic; add unit tests covering both compressible and incompressible template specializations to verify correctness.

gsMatrix<T> gsMaterialMatrixNonlinear<dim,T,matId,comp,mat,imp>::eval3D_pstretch(const index_t patch, const gsMatrix<T> & u, const gsMatrix<T> & z) const

src/gsMaterialMatrixNonlinear.hpp:220

• Stray debug statement left in production code; consider removing the gsDebugVar call or guarding it behind a debug flag.

    gsDebugVar("AAAAA)");