calculate_R0: Calculate the Dengue reproduction number (R_0)
In mrc-ide/drep: Dengue reproduction number calculation

calculate_R0 calculates the dengue reproduction number using the at-equilibrium number of primary, secondary, tertiary and quaternary infections in a population and their relative infectiousness.

1	calculate_R0(FOI, n_j, u_lim, l_lim, phis)

`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.
`phis`	A numeric vector of length = 4 of the relative infectiousness of primary, secondary, tertiary and quaternary dengue infections.