NULL does not import into Stata nicely

[michaelkotrous]

Adding the NULL values corrected problems importing the data into R, but Stata treats it as a string instead of a missing value, which results in variables that need to be treated as integers being treated as strings.

Potential fixes:

1. Add option to aircra.sh that allows user to specify if empty fields should be labeled NULL or left empty.
2. Post Stata snippet in README.md for do file that converts NULL to missing and variables to the correct format for analysis.