Calculate births for each element of place of birth - place of residence stock matrix

This function is predominantly intended to be used within the ffs routines in the migest package.

Usage

birth_mat(b_por = NULL, m2 = NULL, method = "native", non_negative = TRUE)

b_por: Vector of numeric values for births in each place of residence
m2: Matrix of migrant stock totals at time t+1. Rows in the matrix correspond to place of birth and columns to place of residence at time t+1.
method: Character string of either "native" or "proportion" to choose method to distribute births. The "proportion" method assumes the rate of non-migration increase in each place of birth sub-group (native born and all foreign born stocks) is the same. The "native" method ensures that all births (non-migration increases) in stocks belong to the native born population (they do not move straight after birth).
non_negative: Adjust birth matrix calculation to ensure all deductions from m2 will result in positive population counts. On rare occasions when working with international stock data the number of births can exceed the increase in the number of native born population.

Matrix of place of birth by place of residence for new-born’s