pop_dhs: Merging multiple DHS files

This .do file merges multiple DHS's, and keeps memory requirements to a minimum; what it does it quite simple: 1. Loops through all available survey data files (and counts them) 2. For each file, generates variables with missing values if they are not in the file 3. Generates a partial file for each variable extraction 4. Merges all partial files and deletes them What the user needs to do is to: - paste the name of all surveys on top of the file (after local survey_list) - paste the list of all DHS variables needed into the file (after global myvars)