get_ellipse: Retrieve covariance ellipse for single result
In ashiklom/mvtraits: Fit multivariate models to partially missing trait data

Retrieve covariance ellipse for single result

1	get_ellipse(mu, Sigma, xvar, yvar)

`mu`	List of mean coefficient statistics (see `fit_mvnorm()` and `fit_mvnorm_hier()`)
`Sigma`	List of variance-covariance matrix statistics (see `fit_mvnorm()` and `fit_mvnorm_hier()`)
`xvar`	Name of x variable as string (must match row name in `matlist`)
`yvar`	Name of y variable as string (must match column name in `matlist`)