Returns the migration ratio for age interval [Age,Age+AgeInterval) and destination ToPop for a multiple transition migration object MigrationHandle. If AgeInterval is not specified it defaults to 1.0. Migration ratio \(_n c_{\topop x}\) is given by the expression \begin{equation} _n c_{\topop x} = \frac{_n d_{\topop x}}{_n d_x}, \end{equation} where \(_n d_x\) is the number of out-migrations from Age \(x\) for AgeInterval \(n\) and \(_n d_{\topop x}\) is the number of out-migrations to population ToPop.