populationExport function for situation report

[DavidSoSiZoch]

Adding a function that exports all necessary population data for the creation of the situation report.

The function takes as arguments a connection to a SORMAS Postgres db, a start date and an end date.

In the output table each row is a population group by age and gender in a specific district.
The columns are the following variables:
district_id, region_id, sex_population, agegroup_population, population, district_name