Register custom MPI operator for aarch64

[giordano]

I still need to test this, but opening to remind myself to look into this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.46%. Comparing base (2403cc9) to head (aa33cc8).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #287   +/-   ##
=======================================
  Coverage   92.46%   92.46%           
=======================================
  Files           7        7           
  Lines         385      385           
=======================================
  Hits          356      356           
  Misses         29       29           

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[giordano]

Well, now we're testing this on CI on aarch64-darwin, and it seems to be working fine 🙂 CC: @sguillas.

[giordano]

For the record, I ran the tests successfully on both A64FX and Nvidia Grace (besides the fact we're now testing the package on aarch64-darwin, too)