perf: better fitting approximations for spergel and moffat

[beckermr]

I am trying to make these work better and be easier to generate.

[codspeed-hq]

Merging this PR will degrade performance by 46.07%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
❌ 2 regressed benchmarks
✅ 32 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_benchmarks_lanczos_interp[xval-no_conserve_dc-run]	101.4 µs	68.2 µs	+48.77%
⚡	WallTime	test_benchmark_moffat_init[run]	83.1 µs	62.4 µs	+33.07%
❌	WallTime	test_benchmark_spergel_xvalue[run]	1.6 s	2.3 s	-30.21%
❌	WallTime	test_benchmarks_metacal[run]	18.4 ms	34.1 ms	-46.07%

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_{Comparing spergel-calc-max-rad-eff (441abe5) with main (d2f2d94)}