Use new MPI custom ops 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 95.95%. Comparing base (6c659f5) to head (5d6a59a).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           bg/base_min_in_mpi_reduce    #2066      +/-   ##
=============================================================
- Coverage                      96.32%   95.95%   -0.37%     
=============================================================
  Files                            470      470              
  Lines                          37447    37449       +2     
=============================================================
- Hits                           36070    35934     -136     
- Misses                          1377     1515     +138     

Flag	Coverage Δ
unittests	95.95% <100.00%> (-0.37%)	⬇️

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[benegee]

@vchuravy got it working!

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