random structure generation

[bjmorgan]

Adds the ability to generate N random symmetry-inequivalent configurations without full enumeration, useful when the configuration space is too large to enumerate completely.

New Features

• ConfigurationSpace.random_unique_configurations() - generate N random unique configurations
• random_unique_structure_substitutions() - pymatgen interface for random structure generation
• Two sampling modes:
  • degeneracy_weighted (default): high-degeneracy configurations more likely to be sampled
  • uniform: equal probability for each equivalence class
• Reproducible results via seed parameter

Documentation

• New user guide: random_sampling.ipynb

Tests

• Unit tests for ConfigurationSpace.random_unique_configurations() and helpers
• Unit tests for random_unique_structure_substitutions()
• Integration tests for both