Add Neo-Hookean (NH) model and unit tests in FIM

[xiangchenjhu]

Description:

This PR introduces the Neo-Hookean (NH) constitutive model into the FIM inverse modeling framework.**
It extends the current material model options (previously linear and HGO) and enables parameter identification for isotropic nonlinear materials.

1. Implemented calculate_VWS_nh following the same vectorized structure as existing models.
2. Integrated NH into the MaterialModel interface for seamless selection and sensitivity analysis.
3. Verified consistency with the original formulation to ensure correctness.
4. Adds a new dataset for NH model testing (./test_data/NH).
5. Adds an environment file (fim_env.yml) for easy reproducibility of the working environment.

This addition improves the generalizability of FIM by supporting another widely used nonlinear model, broadening applicability for soft matter characterization.

In addition, add unit tests for all three refactor modules to achieve close to 100% coverage.

Running:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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