batch generation of random unique structures

[bjmorgan]

Adds support for batch generation of random unique structures, allowing users to generate structures across multiple runs without duplicates.

New Features

• exclude parameter for ConfigurationSpace.random_unique_configurations() to exclude previously generated configurations
• exclude_file and output_file parameters for random_unique_structure_substitutions() to support batch generation workflows
• Configuration.to_dict() and Configuration.from_dict() methods for serialisation
• save_configurations() and load_configurations() utility functions for JSON file I/O

Documentation

• Updated random_sampling.ipynb with batch generation workflow example