Reinterpret SVector as pointer in mpi reduce

[benegee]

Follow-up to #2054

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (f9c9107) to head (653abff).
Report is 44 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2067   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files         470      470           
  Lines       37447    37462   +15     
=======================================
+ Hits        36070    36085   +15     
  Misses       1377     1377           

Flag	Coverage Δ
unittests	96.32% <100.00%> (+<0.01%)	⬆️

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