The function is a wrapper around derive_vcov_mnl
and
derive_vcov_rpl
and calculates the variance-covariance matrix
of the specified model and design given the priors.
derive_vcov(design_env, model)
An environment containing all the elements necessary to derive the variance-covariance matrix
A string indicating the model for which you wish to derive the variance covariance matrix. Can be either "mnl" or "rpl"
The variance covariance matrix. If the Fisher information matrix is singular, then return NULL