Add posterior analysis functionality with saving and visualization

[Leguark]

Adding test to visualize gempy posterior models

Added a new test module for posterior analysis that visualizes GemPy models using posterior samples. The test loads posterior data from an Arviz netcdf file and plots the mean surface positions across multiple iterations.

Key changes:

• Created new test directory for posterior analysis
• Added functionality to save Arviz data to netcdf for later analysis
• Moved backend initialization before model computation for consistency
• Added visualization of posterior mean surface positions using gempy_viewer
• Enhanced dev logs with notes about saving posteriors and likelihood functions

The test demonstrates how to load posterior samples and visualize their impact on geological models, providing a foundation for more advanced posterior analysis workflows.

[Leguark]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add example files for gravity and petrophysics inversion #9
• Add benchmarking tests and examples generator for performance analysis #8
• Add thickness likelihood module and refactor existing likelihood functions #7
• Add posterior analysis functionality with saving and visualization #6 👈 (View in Graphite)
• Add probabilistic model examples and tests for GemPy #5
• Reorganize gempy_probability package structure and update imports #4
• Update version to 2025.1.0 and refactor version import #3
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.