Check final output is scalar

[SophieL1]

Closes #15

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.05%. Comparing base (a6f3183) to head (5d6e1df).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #16   +/-   ##
=======================================
  Coverage   99.05%   99.05%           
=======================================
  Files          10       10           
  Lines        1483     1484    +1     
=======================================
+ Hits         1469     1470    +1     
  Misses         14       14           

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