Show the parameter correlation matrix with `pwkit.ndshow_gtk3`.

def show_corr(self):
        "Show the parameter correlation matrix with `pwkit.ndshow_gtk3`."
        from .ndshow_gtk3 import view
        d = np.diag(self.covar) ** -0.5
        corr = self.covar * d[np.newaxis,:] * d[:,np.newaxis]
        view(corr, title='Correlation Matrix')