Generates the covariance matrix of the random effects from a sparse representation
loglikD(D_data, gamma, u)
A lower triangular matrix matrix
Named list specifying the covariance matrix D. Usually the return from the member function `get_D_data()` of the covariance class
Vector of covariance parameters specified in order they appear column wise in the functions specified by `func_def`
A realisation of the random effects