added: handle NaN values in measured output ym for all state estimators

[franckgaga]

This is an alternative to #403 essentially implementing the same feature, that is, skipping the correction step when the measured outputs is not available. The advantage compared to #403 is the simplicity in the logic, and a uniform behavior for the MovingHorizonEstimator and all the other state estimators: simply pass a ym vector with NaN values to preparestate! and updatestate! when measurements are not available. Also, assigning nothing to the ym argument in preparestate! and updatestate! functions is equivalent to a vector filled with NaN values.

To summarize, the behavior is:

• MovingHorizonEstimator : skip all the NaN values in the objective function.
• IntenalModel : assign a 0 value in the current estimated stochastic output $\mathbf{\hat{y}_s}(k)$ for all NaN values in ym argument.
• All the Kalman filters and Luenberger: skip the correction step altogether if there is at least one NaN in ym argument. Note that there are other and more refine approaches to this, but skipping the correction step is a good and simple method for this.

A customize @warn is also printed to mention explicitly what is being done to the user.

This effectively implement the functionality mentioned by @baggepinnen in #89.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.38710% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.72%. Comparing base (bc0f8e9) to head (690296f).

Files with missing lines	Patch %	Lines
src/estimator/internal_model.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #404   +/-   ##
=======================================
  Coverage   98.71%   98.72%           
=======================================
  Files          27       27           
  Lines        5703     5722   +19     
=======================================
+ Hits         5630     5649   +19     
  Misses         73       73           

☔ View full report in Codecov by Harness.
📢 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.