Make access to matrices of the Kalman filter easier

[spmvg]

Darts uses the Kalman filter to bootstrap a dataset based on the fitted covariances (see #1 (comment)). To do this, access to undocumented (but not private) attributes is necessary: see https://github.com/unit8co/darts/pull/692/files.

FYI I thought about integrating the dataset generation method that Darts uses into nfoursid, but I decided I could better provide the tools + documentation necessary.

• Determine which attributes should be documented, and which attributes should get an easier API (for example: we ideally don't want to do ...[-1] in production code).
• Implement the documentation and easier API