Improve gpu update/compare interface

[richfitz]

No reason why we can't put the excessively ugly arguments into a struct:

void update_gpu<variable>(size_t step,
                          const dust::cuda::interleaved<variable::real_type> state,
                          dust::cuda::interleaved<int> internal_int,
                          dust::cuda::interleaved<variable::real_type> internal_real,
                          const int * shared_int,
                          const variable::real_type * shared_real,
                          variable::rng_state_type& rng_state,
                          dust::cuda::interleaved<variable::real_type> state_next);

could become

void update_gpu<variable>(size_t step, dust::cuda::model_state<sirs>);

with

template <typename T>
struct model_state {
  using real_type = typename T::real_type;
  const dust::cuda::interleaved<real_type> state;
  dust::cuda::interleaved<int> internal_int;
  dust::cuda::interleaved<real_type> internal_real;
  const int * shared_int;
  const real_type * shared_real;
  variable::rng_state_type& rng_state;
  dust::cuda::interleaved<real_type> state_next);
}

Then update the body to access these