sur_sample_cov_helper_cpp: Helper function to sample covariance
In surbayes: Bayesian Analysis of Seemingly Unrelated Regression Models

This function is called by sur_sample_cov_cpp. It samples the covariance matrix of a SUR

1	sur_sample_cov_helper_cpp(Y, Xlist, n, J, pj, sigma11, r1)

`Y`	A `matrix`, each column a `vector` of responses
`Xlist`	A `list`, each element a design `matrix`
`n`	Integer giving number of observations
`J`	Integer giving number of endpoints
`pj`	A `vector` giving number of covariates per endpoint
`sigma11`	A scalar giving a draw for the (1,1) component of the covariance matrix
`r1`	A `vector` of residuals for the first endpoint's regression