calculate_cases: Calculate the number of per capita dengue cases.
In mrc-ide/drep: Dengue reproduction number calculation

calculate_cases calculates the number of per capita annual dengue symptomatic infections (cases) in a human population, given a force of infection.

1	calculate_cases(FOI, n_j, u_lim, l_lim, weights_vec)

`FOI`	The force of infection value. Numeric.
`n_j`	A numeric vector of the proportion of individuals in each age group in the human population.
`u_lim`	A numeric vector of the upper age limits of the population age groups.
`l_lim`	A numeric vector of the lower age limits of the population age groups.
`weights_vec`	A numeric vector of length = 4 of the proportions of primary, secondary, tertiary and quaternary infections which are symptomatic.