feat: add benchmark and test of flux frac for interp images

[beckermr]

This PR adds a test and benchmark for the _flux_frac routine used by the interpolated image code. It is in preparation for testing a version without the fori_loop.

[codspeed-hq]

CodSpeed Performance Report

Merging #133 will not alter performance

_{Comparing blah-blah (600cad2) with main (5db44f1)}

Summary

✅ 23 untouched benchmarks
🆕 2 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	test_benchmark_interpimage_flux_frac[compile]	N/A	280.4 ms	N/A
🆕	test_benchmark_interpimage_flux_frac[run]	N/A	1.9 ms	N/A