Add robust linear Gaussian update

[FlorianPfaff]

Summary

Backports the reusable robust linear-Gaussian update machinery from the RaFT-UAV work into PyRecEst.

Adds:

• linear_gaussian_update_robust(...)
• NIS / innovation statistics helpers
• Student-t, Huber, and NIS-inflation covariance scaling helpers
• KalmanFilter.update_linear_robust(...)
• KalmanFilter.update_model_robust(...)
• regression tests for scale behavior, outlier downweighting, and the class wrapper

The standard linear_gaussian_update(...) and KalmanFilter.update_linear(...) behavior remain unchanged.

Validation

Locally prepared patch parsed successfully with py_compile for modified files and the new test file. Full test execution was not run in this environment.