ci: add more compilers, mac build, and openmpi to ci suite

[caitlinross]

Adding a more comprehensive suite of builds to CI. This now includes Mac and Linux (Ubuntu) builds, clang and gcc builds, and MPICH and OpenMPI builds.

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Checklist

• Builds cleanly with -Wall and -Wextra
• CI is green
• Added a changelog fragment under Documentation/dev/, unless the change is invisible to anyone outside the PR (test refactors, internal renames, comment-only tweaks)
• Confirmed nothing in CODES breaks. CI has a minimal CODES build that should be green before merging.
• For new features: blog post on the ROSS website, with link in this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.75%. Comparing base (9b6ccb1) to head (7f9914c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #210   +/-   ##
=======================================
  Coverage   48.75%   48.75%           
=======================================
  Files          41       41           
  Lines        4816     4816           
  Branches      860      860           
=======================================
  Hits         2348     2348           
  Misses       2463     2463           
  Partials        5        5           

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[caitlinross]

The test failure in the codes-contract. It's unrelated to the changes in this PR; it seems that the ping pong tests are still flaky.