Clean-up solid materials

[lauraengelhardt]

Description and Context

Some material related clean-ups I came along.
Please see the commit messages for a description of the changes.

[lauraengelhardt]

Maybe someone could request a copilot review as well, I ran out of credits ;)

[Copilot]

Pull request overview

This PR performs cleanup and interface simplification around solid/hyperelastic material summands, removing unused APIs (e.g., varying density / AAA setup / Young’s modulus helper paths) and moving several simple shear-modulus implementations inline into headers.

Changes:

• Remove unused/legacy material APIs (varying_density, setup_aaa, Young’s-modulus aggregation helpers).
• Inline add_shear_mod implementations for multiple elastic summands and drop redundant .cpp definitions.
• Minor modernizations/cleanup in ElastHyper (range-for, [[nodiscard]], formatting/consistency tweaks).

Reviewed changes

Copilot reviewed 23 out of 24 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/membrane/4C_membrane_evaluate.cpp	Removes runtime check for varying density in membrane mass-matrix assembly.
src/mat/elast/4C_mat_elast_summand.hpp	Removes unused AAA setup hook; changes base add_shear_mod behavior/message.
src/mat/elast/4C_mat_elast_summand.cpp	Removes now-redundant base add_shear_mod definition; trivial pack/unpack cleanup.
src/mat/elast/4C_mat_elast_isovarga.hpp	Inlines add_shear_mod implementation.
src/mat/elast/4C_mat_elast_isovarga.cpp	Removes out-of-line add_shear_mod implementation.
src/mat/elast/4C_mat_elast_isoneohooke.hpp	Inlines add_shear_mod; small formatting tweak.
src/mat/elast/4C_mat_elast_isoneohooke.cpp	Removes out-of-line add_shear_mod implementation.
src/mat/elast/4C_mat_elast_coupvarga.hpp	Inlines add_shear_mod implementation.
src/mat/elast/4C_mat_elast_coupvarga.cpp	Removes out-of-line add_shear_mod implementation.
src/mat/elast/4C_mat_elast_coupSaintVenantKirchhoff.hpp	Removes unused Young’s-modulus equivalent helper.
src/mat/elast/4C_mat_elast_coupneohooke.hpp	Implements add_shear_mod inline (replacing prior Young’s helper usage).
src/mat/elast/4C_mat_elast_coupneohooke.cpp	Removes out-of-line add_shear_mod implementation.
src/mat/elast/4C_mat_elast_couplogneohooke.hpp	Inlines add_shear_mod implementation.
src/mat/elast/4C_mat_elast_couplogneohooke.cpp	Removes out-of-line add_shear_mod implementation.
src/mat/elast/4C_mat_elast_couplogmixneohooke.hpp	Inlines add_shear_mod implementation.
src/mat/elast/4C_mat_elast_couplogmixneohooke.cpp	Removes out-of-line add_shear_mod implementation.
src/mat/elast/4C_mat_elast_coupblatzko.hpp	Removes unused Young’s-modulus equivalent helper.
src/mat/elast/4C_mat_elast_coupanisoneohooke_VarProp.hpp	Removes unused AAA setup hook override.
src/mat/elast/4C_mat_elast_anisoactivestress_evolution.hpp	Removes unused AAA setup hook override.
src/mat/4C_mat_so3_material.hpp	Removes unused varying_density() API from the base material interface.
src/mat/4C_mat_elasthyper.hpp	Removes unused APIs and adds [[nodiscard]] to several accessors.
src/mat/4C_mat_elasthyper.cpp	Constructor/unpack cleanup and error message updates (but introduces invalid format strings).
src/mat/4C_mat_aaaneohooke.hpp	Removes unused shear-modulus helper (non-override).
src/mat/4C_mat_aaaneohooke.cpp	Whitespace/format-only cleanup.