message: need mpi header
diff: #include <vector>
  #include <memory>
  +#ifdef STAN_MPI
  +#include <stan/math/prim/arr/functor/mpi_cluster.hpp>
  +#endif
  +
  // forward declaration for function defined in another translation unit
  stan::model::model_base& new_model(stan::io::var_context& data_context,
  