This function takes a dataframe and creates binary missing indicator variable. This can be realized with two
different approaches:
Approach 1 (drop_NA_col = FALSE): creates a binary missing indicator variable for partially observed variables and retains both original and indicator variables.
Approach 2 (drop_NA_col = TRUE): creates a binary missing indicator variable for partially observed variables and only retains indicator variables (and drops the original variables).
Important: Make sure you have your variables format correct and avoid to include variables like ID variables, ZIP codes, dates, etc.