Create new column with first row of each id

General Usage
2 
julia> transform!(groupby(df, :id), :x1 => first)
9×4 DataFrame
 Row │ id     x1     x2     x1_first 
     │ Int64  Int64  Int64  Int64    
─────┼───────────────────────────────
   1 │     1      1     10         1
   2 │     1      4     13         1
   3 │     1      7     16         1
   4 │     2      2     11         2
   5 │     2      5     14         2
   6 │     2      8     17         2
   7 │     3      3     12         3
   8 │     3      6     15         3
   9 │     3      9     18         3
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