FEAT make NARX accept multi-output

[MatthewSZhang]

Implement #53

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d4306c4) to head (b389610).
Report is 52 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #54    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            3         6     +3     
  Lines          110       476   +366     
==========================================
+ Hits           110       476   +366     

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