feat: add benchmarks for class init

[beckermr]

This PR adds benchmarks for the Spergel and Gaussian inits. I will use it to compare against optimizations in a future PR.

[codspeed-hq]

CodSpeed Performance Report

Merging #128 will not alter performance

_{Comparing bench-spergel-hlr (f56e1ab) with main (1e0a681)}

Summary

✅ 17 untouched benchmarks
🆕 4 new benchmarks

Benchmarks breakdown

	Benchmark	main	bench-spergel-hlr	Change
🆕	test_benchmark_gaussian_init[compile]	N/A	16 ms	N/A
🆕	test_benchmark_gaussian_init[run]	N/A	85.3 µs	N/A
🆕	test_benchmark_spergel_init[compile]	N/A	1.9 s	N/A
🆕	test_benchmark_spergel_init[run]	N/A	1.5 ms	N/A

[beckermr]

I'm going to merge this so I can work on the improvements.