Add SIMD vectorization to mapreduce_neighbor_unsafe

[efaulhaber]

This PR adds a kwarg simd to mapreduce_neighbor_unsafe. Enabling this option adds a loop header that forces the inner loop to SIMD-vectorize.

A simple @simd doesn't trigger proper vectorization for TLSPH in my tests, so (unfortunately) we need this extra loop header. In situations where vectorization is not possible (e.g. when the loop contains branching or bounds checks), we get a warning, which is why this is not enabled by default. I would prefer a silent failure, so mapreduce_neighbor_unsafe always tries to vectorize, with an option to enable the warning for debugging, but this doesn't seem to be possible, as the warnings are coming from deep within Julia.

Note that vectorization doesn't work with bounds checks, so there is no point in adding this kwarg to mapreduce_neighbor.

This kwarg is intentionally undocumented, so we can test it first before we decide on a final public API.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.40%. Comparing base (78be5a6) to head (f8de39d).

Files with missing lines	Patch %	Lines
src/nhs_precomputed.jl	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
- Coverage   84.59%   84.40%   -0.20%     
==========================================
  Files          15       15              
  Lines         792      795       +3     
==========================================
+ Hits          670      671       +1     
- Misses        122      124       +2     

Flag	Coverage Δ
unit	84.40% <60.00%> (-0.20%)	⬇️

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