Kalman filter log-likelihood

[MichaelClerx]

Fixes #433

[ben18785]

@martinjrobins Interesting to compare this to AR1 model as think they're the same although estimation approach is different (you use the Kalman filter to compute the log-likelihood, I use another approach).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (82471f9) to head (d8ac869).
⚠️ Report is 1965 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #713   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           68        68           
  Lines         7230      7230           
=========================================
  Hits          7230      7230           

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[MichaelClerx]

Fixed the git mess I made here. Please delete local copy and re-check out before continuing work on this

[ben18785]

@martinjrobins this is an oldie but a goodie. Think we might as well try and include it, so long as we have some tests written...