Numerical Benchmarking of GREAT Cell Experiments

This merge request introduces the first Jupyter notebook for the GreatCell benchmark suite, part of the DECOVALEX-2027 project.

The notebook focuses on simulating mechanical deformation of intact and fractured samples using the SmallDeformation and SMALL_DEFORMATION_WITH_LIE process in OGS. This is the initial step, and future notebooks will extend this work to include hydro-mechanical coupling. A similar structure will be used later for the variational phase-field approach (VPF), based on submitted manuscript

To keep things clear and organized, mesh generation, simulation running (with different loading conditions and materials), and post-processing are separated into their own Python classes. These reusable parts will be used in future notebooks for hydromechanical and VPF.