Fixed a bug with the `gaussian` approach not using the user-provided … (#353)

* Fixed a bug with the "gaussian" approach not using the user-provided values for the gaussian.mu and gaussian.cov_mat arguments, and added a test function. Discuss with Martin what to do with the 2nd and 3rd function arguments, as they are unnecessary, and the roxygen2 documentation.

* adds insert_default call for consistency

* change to expect_equal to avoid potential round off errors in the future

* linting

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Co-authored-by: Martin <jullum@nr.no>