Convention for output of tuples

In the develop branch, tuples generated by Stan programs are one-dimensional arrays of tuples. This can make it difficult to access elements of tuples. For example, consider the following test.

def test_tuple_out() -> None:
    stan = os.path.join(DATAFILES_PATH, 'tuple_out.stan')
    model = CmdStanModel(stan_file=stan)
    a = np.random.normal(0, 1, (5, 5))
    b = np.random.normal(0, 1, 3)
    fit = model.sample({"a": a, "b": b}, fixed_param=True, chains=1,
                       iter_sampling=20, iter_warmup=1, sig_figs=18)
    np.testing.assert_allclose(a, fit.stan_variable("c")[0][0])
    np.testing.assert_allclose(b, fit.stan_variable("c")[0][1])

// tuple_out.stan
data {
    matrix [5, 5] a;
    vector [3] b;
}

generated quantities {
    tuple(matrix[5, 5], vector[3]) c;
    c.1 = a;
    c.2 = b;
}

Then accessing c.1 samples is only possible through list comprehension (there may be some fancy indexing I'm not familiar with).

>>> fit.c.shape
(20,)
>>> fit.c[:, 0]
IndexError: too many indices for array: array is 1-dimensional, but 2 were indexed

Would it make sense to return a tuple of arrays rather than an array of tuples? This would allow accessing samples more easily, e.g., in the above example we'd get

>>> fit.c[0].shape
(20, 5, 5)

This would however go against the convention that the first index refers to samples.

Start with the tuple_out.stan example and the test_tuple_out test shown in the issue, then inspect fit.stan_variable("c") and fit.c behavior. Compare array-of-tuples and tuple-of-arrays layouts, and define the resulting sample indexing convention before updating the relevant tests and documentation.