ENH speed up grad computation via Cython

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: FEAT adopt Cython to speed up evaluation of NARX prediction #56
• A description of the changes: use Cython to speed up grad computation for NARX

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (84b2f83) to head (b96534e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #71   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          603       582   -21     
=========================================
- Hits           603       582   -21     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[MatthewSZhang]

#70 Fix time-narx, time-h, time-eta, nogil-h, and nogil-eta put all code in setup and timing empty

[MatthewSZhang]

On M1 chip, pixi run time-narx outputs

• 6.3 sec without Cython speedup
• 1.4 sec with Cython speedup