Simplify and generalise `get_exposure_ages`

[ben18785]

As I understand it, get_exposure_ages just generates the ages of individuals within a serological sample. At the moment, it does the following:

• determines the minimum birth year in the sample via min(serodata$birthyear)
• determines the survey year by looking at the first row of the serodata and subtracting 1
• creates a vector of ages from the minimum birth year to the year before the survey

But what if the data a user supplies does not have samples for all ages? Also, the requirements on the user are somewhat implicit, as the data frame needs to have columns named in a certain way. I'd propose changing this to:

create_exposure_ages <- function(birthyears, survey_year)

where birthyears is a vector of birth years.

[ekamau]

what if the user has data as age and not birth year ... is that step skipped? or should the user calculate birth year, externally?

[ntorresd]

Outdated issue.