SSM get_param

[mredaelli]

#4 R CMD check failed before I touched it, so not sure what to do :P

• [X] if suggesting code changes or improvements, open an issue first
• [X] for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• [X] for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• [X] if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• [ ] add code or new test files to /tests for any new functionality or bug fix
• [ ] make sure R CMD check runs without error before submitting the PR