Add 1D mode and FFT updates

[darkviruzz]

Summary

• introduce optional mode parameter to rcwa.obj for choosing 1-D or 2-D operation
• enforce lattice vector and grid size restrictions in 1-D mode
• add 1‑D implementations of get_conv, get_fft and get_ifft
• extend lattice utility functions to generate and use 1‑D G sets
• update backend with 1‑D FFT helpers
• document 1‑D usage and add binary grating example
• expand test suite for new functionality

Testing

• make lint
• make test

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https://chatgpt.com/codex/tasks/task_e_68557670393883299a4c0ec9c9ff2234