Add get_levels_polars() and refactor atomic data readers

[lukeshingles]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging #209 will improve performances by 43.91%

_{Comparing refactor_get_transitiondata (c02d40f) with main (9d01080)}

Summary

⚡ 2 improvements
✅ 21 untouched benchmarks

🆕 1 new benchmarks

Benchmarks breakdown

	Benchmark	main	refactor_get_transitiondata	Change
⚡	test_get_levels	9.8 s	6.8 s	+43.91%
🆕	test_get_levels_polars	N/A	6.7 s	N/A
⚡	test_nltepops	5.6 s	4.3 s	+28.5%