Add Linear Nk OCCRI method + ISDF

[kosm6966]

##Summary

• Add OCCRI method for efficient exact exchange evaluation in periodic systems with significant algorithmic improvements.
• Performance is similar to but typically faster than existing occri method
• Designed to integrate with isdfx
• Provide memory-efficient implementations with MO blocking following PySCF patterns
• Implement ISDFX (Interpolative Separable Density Fitting eXchange) module with comprehensive testing and examples
• Exchange build is linear with N_k as shown by Example 2
• Default threshold is set and provides < 50 μHa error for all test systems
• Include comprehensive test suites and educational examples demonstrating usage and performance

Features

• OCCRI: Exchange matrix evaluation using occupied orbital resolution of identity
• ISDFX: Tensor hypercontraction-based exchange with interpolative separable density fitting
• Support for RHF, UHF, RKS, UKS and k-point variants (KRHF, KUHF, KUKS, KRKS)
• Memory management with automatic MO blocking for large systems
• Comprehensive utility functions separated from object methods for better code organization
• Natural orbital construction and exchange matrix building utilities

Performance & Accuracy

• ISDFX provides near-FFTDF accuracy (typically <50 μHa energy differences)
• Scaling analysis shows sub-quadratic performance with k-point sampling
• Memory-efficient MO blocking prevents memory overflow for large systems
• Proper handling of asymmetric k-point meshes and complex conjugation

Testing & Examples

• 84 comprehensive tests covering energy accuracy, k-point handling, and edge cases
• Energy comparison tests with 50 μHa tolerance vs FFTDF reference
• Asymmetric k-point tests to catch symmetry and conjugation bugs
• Educational examples: basic usage, AFM initial guess setup, scaling analysis
• Integration with existing run_tests.py framework with --perf flag support

Code Quality

• Follows PySCF coding conventions with comprehensive docstrings
• Separation of pure algorithms from object methods for better maintainability
• Formatted with Black, isort, and flake8
• Extensive error handling and input validation
• Memory cleanup following PySCF patterns

Test plan

• All 84 tests pass including energy accuracy and performance tests
• Examples demonstrate proper usage, AFM setup, and scaling behavior
• Code quality checks pass (Black, isort, flake8)
• Memory management validated with MO blocking implementation
• K-point handling thoroughly tested with asymmetric meshes

[jeanwsr]

Is it better to put these codes in pbc/df?

[kosm6966]

Is it better to put these codes in pbc/df?

I'm happy to move these here. Just let me know your preference. Thanks!