MNT output proper constant when some outputs only have intercepts

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: Use mean value instead of 0 when no poly term assigned to an output #57 Update warning message from 0 to intercept #58 BUG narx grad stuck when no dynamic part #72
• A description of the changes

1. Output intercept when no poly term is assign to that output. It should be mean value for that output.
2. Update the warning message. We allow no poly term for some outputs, but at least one poly term should be assigned to one output.
3. For Jacobian of NARX, it should skip to compute dynamic part when no y in terms.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (84b2f83) to head (727a676).
Report is 5 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines          603       583   -20     
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- Hits           603       583   -20     

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