Update kernel benchmark

[charleskawczynski]

I think the original kernel benchmark I added was very memory bound (from the output array), as a result performance changes would be hidden by the bandwidth of loading the output array.

What I think we're really interested in here is flops inside a realistic kernel, and memory reads necessary for a single thermo state.

I've updated the kernel benchmark to address the first part (flops), and we should write a test for the memory reads necessary for a single thermo state, however, it will require a bit more work, and it'll be somewhat brittle to compare a broadcast call vs a custom kernel. Interesting, the kernel in this PR seems to be about 3x faster than the broadcast function call, I'll need to dig into that later.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2735f2) 92.91% compared to head (ca56ec8) 92.91%.

Additional details and impacted files

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  Coverage   92.91%   92.91%           
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  Files          10       10           
  Lines        1143     1143           
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  Hits         1062     1062           
  Misses         81       81           

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