10.3 Fitting a GP ARD update for array

[spinkney]

In https://mc-stan.org/docs/stan-users-guide/fit-gp.html#automatic-relevance-determination

The function signature is written

  matrix L_cov_exp_quad_ARD(vector[] x,
                            real alpha,
                            vector rho,
                            real delta)

But that vector[] x now needs to be array[] vector x.