Enable optional prescribed initial prestretch

[tuchpaul]

This PR introduces support for a predefined prestretch as the starting point for the IterativePrestress algorithm.
The predefined prestretch can be provided as an interpolated input field. To make this initial prestretch optional, the possibility to set default values for both the interpolated input field and the selection have been implemented.

The new test cases covers the new feature and represents the prestressing performed in paragraph 2.6 in
https://doi.org/10.1016/j.jmbbm.2023.105733.

[Copilot]

Pull request overview

This PR adds support for an optional, user-provided initial prestretch tensor as the starting point for the IterativePrestress strategy, with identity as a default when not specified.

Changes:

• Add PRESTRETCH as an optional InterpolatedInputField<SymmetricTensor<...>> parameter for MIX_Prestress_Strategy_Iterative, defaulting to the identity tensor.
• Extend Core::IO::InputSpecBuilders to support optional selections and default values for (interpolated) input fields.
• Add value_type alias to generalized input fields to enable default-value plumbing in specs.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/mixture/src/4C_mixture_prestress_strategy_iterative.hpp	Adds a stored initial prestretch input field parameter to the iterative prestress strategy parameters.
src/mixture/src/4C_mixture_prestress_strategy_iterative.cpp	Uses the initial prestretch field as the starting value in evaluate_prestress().
src/global_legacy_module/4C_global_legacy_module_validmaterials.cpp	Registers PRESTRETCH as an optional material input with identity default for the iterative prestress strategy.
src/core/io/src/4C_io_input_spec_builders.hpp	Adds optional/required + defaulting support for selections and (interpolated) input fields.
src/core/io/src/4C_io_input_field.hpp	Exposes value_type needed by spec builders’ default-value handling.