get_all_df.RdColumns missing in some results set are added with NA values.
get_all_df( path = "../inst/extdata/", .fun = get_summary_df, col_names = get_col_names("syntactic_summary") )
| path | file path to directory containing files to be read.  | 
    
|---|---|
| .fun | the function object to be used to collect combined results.  | 
    
| col_names | character vector of column names in the order they are to appear.  |