Add macOS to GitHub Actions

[RMeli]

No description provided.

[codecov]

Codecov Report

Merging #50 (8fe3693) into master (6a0e708) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   98.02%   98.02%   -0.01%     
==========================================
  Files          21       21              
  Lines        2484     2584     +100     
==========================================
+ Hits         2435     2533      +98     
- Misses         49       51       +2     

Impacted Files	Coverage Δ
include/libirc/atom.h	100.00% <0.00%> (ø)
src/test/irc_test.cpp	100.00% <0.00%> (ø)
src/test/linalg_test.cpp	100.00% <0.00%> (ø)
src/test/wilson_test.cpp	100.00% <0.00%> (ø)
src/test/connectivity_test.cpp	100.00% <0.00%> (ø)
src/test/periodic_table_test.cpp	100.00% <0.00%> (ø)
include/libirc/connectivity.h	96.73% <0.00%> (+0.07%)	⬆️
include/libirc/wilson.h	95.81% <0.00%> (+0.08%)	⬆️
include/libirc/irc.h	83.44% <0.00%> (+0.24%)	⬆️
include/libirc/transformation.h	97.05% <0.00%> (+1.40%)	⬆️

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[RMeli]

Eigen needs to be pinned to <3.4 for the time being (#51).

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Installing an old formula with homebrew:

brew tap-new $USER/local-${{matrix.lamacos}}
brew extract --version ${{matrix.vmacos}} ${{matrix.lamacos}} $USER/local-${{matrix.lamacos}}
brew install $USER/local-${{matrix.lamacos}}/${{matrix.lamacos}}@${{matrix.vmacos}}