Model customisation

TODO:

To change this we can call g3_parameterized() ourself with different parameters:

attr(suppressWarnings(g3_to_tmb(list(
    g3a_naturalmortality(
        fish,
        g3a_naturalmortality_exp(g3_parameterized("M", by_stock = TRUE, by_age = FALSE)))
    ))), 'parameter_template')