Enhance post_process_mesh for improved cluster filtering

[filippocastelli]

This pull request improves the mesh post-processing utility by making the filtering of mesh clusters more robust and configurable. The main enhancement is the ability to specify a minimum cluster size and to ensure that the number of clusters kept is always valid, which helps prevent errors and improves mesh quality.

Improvements to mesh post-processing:

• The post_process_mesh function now accepts a min_cluster_size parameter, allowing users to filter out clusters smaller than a specified threshold.
• The function clamps the number of clusters to keep (clusters_to_keep) to a valid range based on the actual number of clusters in the mesh, preventing out-of-bounds errors.
• Added logic to handle cases where no clusters are found, returning the mesh unchanged and printing a message for clarity.
• Improved code clarity by removing unused imports and moving the copy import to the top of the file.