new ManualEstimator tests and minor doc corrections

[franckgaga]

Added an integration test with ManualEstimator with an external state estimators. The comparison is done with the default setup, that is the built-in SteadyKalmanFilter for linear models, and UnscentedKalmanFilter, for nonlinear models.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.92%. Comparing base (44a5589) to head (408701b).
⚠️ Report is 775 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #210   +/-   ##
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  Coverage   98.92%   98.92%           
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  Files          26       26           
  Lines        4285     4285           
=======================================
  Hits         4239     4239           
  Misses         46       46           

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