Adds age group marker to serosurvey
add_age_group_to_serosurvey(serosurvey)serosurvey with additional column specifying age group marker
defined as the mean floor between age_min and age_max
survey_yearYear in which the survey took place (only needed to plot time models)
age_minFloor value of the average between age_min and age_max
age_maxThe size of the sample
n_sampleNumber of samples for each age group
n_seropositiveNumber of positive samples for each age group