Use fully parallel atomic initialization

[efaulhaber]

Moving away from serial initialization with FullGridCellList. I found that this is actually faster for medium to large problems than the incremental update, and it's also much simpler, so I made it the default and added a separate update mode for incremental update.

I'll have to investigate this in actual simulations and benchmark GPUs before continuing here.

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.57%. Comparing base (fde9a79) to head (532a64f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nhs_grid.jl	81.25%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   88.15%   84.57%   -3.59%     
==========================================
  Files          15       15              
  Lines         498      512      +14     
==========================================
- Hits          439      433       -6     
- Misses         59       79      +20     

Flag	Coverage Δ
unit	84.57% <82.35%> (-3.59%)	⬇️

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[sloede]

Nice speedup!