Xiangyu/physical closure

[Xiangyu-Hu]

The material model now is generalized to physical closure.
Under this, material model (basically equation of state for fluid, and constitute relation for solid)
is considered as the base model, and viscosity is split and considered as an auxiliary model.

By such generalization, single species and multi-species diffusion are considered as auxiliary models.
However, I have not decide whether reaction system should be one part of closure or still belongs to diffusion reaction, as their relations are still not clarified.

As a straightforward extension, one can include turbulence models into the closure too.

The follows are copilot generated summary:

This pull request includes several changes aimed at improving the material handling and diffusion dynamics in the SPHinXsys library. The most important changes include the introduction of closures, removal of weakly compressible fluid materials, and enhancements to diffusion dynamics.

Material Handling Improvements:

• modules/structural_simulation/structural_simulation_class.cpp: Changed material assignment to use defineMaterial<SaintVenantKirchhoffSolid> instead of assignMaterial.
• src/shared/bodies/base_body.h: Introduced a new defineClosure method for defining closures and removed the assignMaterial method.

Removal of Weakly Compressible Fluid Materials:

• src/shared/materials/weakly_compressible_fluid.cpp and src/shared/materials/weakly_compressible_fluid.h: Removed the entire implementation of weakly compressible fluid materials and their associated classes.
  Only the base model is kept. [1] [2]

Diffusion Dynamics Enhancements:

• src/shared/particle_dynamics/diffusion_reaction_dynamics/diffusion_dynamics.cpp: Added a new implementation for GetDiffusionTimeStepSize to compute time step size based on diffusion coefficient and particle smoothing length.
• src/shared/particle_dynamics/diffusion_reaction_dynamics/diffusion_dynamics.h: Simplified the GetDiffusionTimeStepSize class and modified DiffusionRelaxation to improve species registration and update methods. [1] [2]

Other Changes:

• src/shared/io_system/io_environment.cpp and src/shared/io_system/io_environment.h: Changed the defineParameterizationIO method to return a pointer instead of a reference. [1] [2]
• src/shared/common/common_functors.h: Introduced a new ConstructArgs template struct for handling constructor arguments.