Function that computes rates as the ratio between the observed migration value and the total population (for a given age, gender, region and ethnicity).
This function should work for both the emigration and immigration rates, important for #28 and #29.
Closes #27
Function that computes rates as the ratio between the observed migration value and the total population (for a given age, gender, region and ethnicity).
This function should work for both the emigration and immigration rates, important for #28 and #29.