Convert Dataframe column containing strings "A+", "A", "A-" ..."D+", "D", "D-" to integers 12,11,10 ... 3,2,1

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julia> using DataFramesMeta; 

julia> grades_df = DataFrame(grade = ["A+", "B", "B-"]);

julia> grades_mapping = Dict("A+" => 12, "A" => 11, "A-" => 10, "B+" => 9, "B" => 8, "B-" => 7);

julia> @rtransform grades_df :grade_num = grades_mapping[:grade]
3×2 DataFrame
 Row │ grade   grade_num
     │ String  Int64
─────┼───────────────────
   1 │ A+             12
   2 │ B               8
   3 │ B-              7
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