Use new MPI custom ops in mpi reduce

[benegee]

Follow-up to #2054

[github-actions[bot]]

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 96.77%. Comparing base (cec533a) to head (65385b1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2066   +/-   ##
=======================================
  Coverage   96.76%   96.77%           
=======================================
  Files         521      521           
  Lines       42456    42458    +2     
=======================================
+ Hits        41082    41085    +3     
+ Misses       1374     1373    -1     

Flag	Coverage Δ
unittests	96.77% <100.00%> (+<0.01%)	:arrow_up:

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[benegee]

@vchuravy got it working!

Despite one single very strange error involving vectorization... see CI.