Add test for spherical advection in Cartesian coordinates

[tristanmontoya]

Addresses #28.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.78%. Comparing base (d8b0d31) to head (c53fcb9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #30       +/-   ##
===========================================
+ Coverage   50.90%   81.78%   +30.87%     
===========================================
  Files           8        8               
  Lines         827      829        +2     
===========================================
+ Hits          421      678      +257     
+ Misses        406      151      -255     

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[amrueda]

Looks great to me!
Many thanks, @tristanmontoya !