Get the prior density of sediment rates

        Returns
        -------
        y : ndarray
            Array giving the density.
        x : ndarray
            Array of sediment accumulation values (yr/cm) over which the density was evaluated.

def prior_sediment_rate(*args, **kwargs):
        """Get the prior density of sediment rates

        Returns
        -------
        y : ndarray
            Array giving the density.
        x : ndarray
            Array of sediment accumulation values (yr/cm) over which the density was evaluated.
        """
        # PlotAccPrior @ Bacon.R ln 113 -> ln 1097-1115
        # alpha = acc_shape, beta = acc_shape / acc_mean
        # TODO(brews): Check that these stats are correctly translated to scipy.stats distribs.
        acc_mean = kwargs['acc_mean']
        acc_shape = kwargs['acc_shape']
        x = np.linspace(0, 6 * np.max(acc_mean), 100)
        y = stats.gamma.pdf(x, a=acc_shape,
                            scale=1 / (acc_shape/acc_mean))
        return y, x