Takes a delta dataset and adjusts the outcome variable by adding the
corresponding delta.
     
    
    Usage
    apply_delta(data, delta = NULL, group = NULL, outcome = NULL)
 
     
    
    Arguments
- data
 
data.frame which will have its outcome column adjusted.
 
- delta
 
data.frame (must contain a column called delta).
 
- group
 
character vector of variables in both data and delta that will be used
to merge the 2 data.frames together by.
 
- outcome
 
character, name of the outcome variable in data.