weighted_rowsums() is used to get the row-wise sums of a matrix after
column-wise multiplication by a vector of weights.
interest_accumulation() is used to calculate the new principal after
assuming an interest rate and with some external contribution. Used in
get_fiscal_costs() when the borrowed principal to fund pandemic response
both compounds over time, and increases due to fresh borrowing in each
timestep.
annual_rate_daily() is used to convert an annual rate of interest to a
daily rate, for use in interest_accumulation() and eventually in
get_fiscal_costs().