Add GroupBy function and example run #16
                
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Checklist for PR
[x] Link to corresponding Issue. 8
[x] Tests covering all the code branches.
[x] Validations for all the argument's type. This includes the functions passed as arguments.
[x] A Comment on how the function works and what are the different validations in the function.
[x] An ExampleTest of the function.
[x] README.md should be updated about the function.