validation for missing programme data

[jeffeaton]

Write a check for whether any area/sex/age group combinations have missing programme data. The check is slightly complex because the areas are represented by ad hoc aggregations

Checks:

1. Require that every stratification is represented by a row in the data.
2. Provide an option whether to allow NA values or not. Probably a on model option page for this.

Steps:

1. Filter to the finest area / sex / age group stratification.
2. Map the observed aggregations to the fine stratifications.
3. anti_join() to find any that are missing

Report back as (1) entire area missing data, (2) area / sex / age stratifications missing data.