Initialize unstructured covariance model parameters with empirical covariance

[danielinteractive]

See https://github.com/openpharma/mmrm/blob/main/design/unstructured_optimization.Rmd

• Add unit test for all functions (including internal ones)
• Adjust style for Roxygen documentation
• title style for titles
• blank lines for readability between sections
• parentheses for type with \cr afterwards
• At least one sentence description before arguments
• add a news entry