Create shifted_seymanur_sahin.py

[shnseymanr]

Week 3 homework: function 'shifted' calculates the difference between the mean and median in percantage

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• I have read the [CONTRIBUTING]
• I have performed a self-review of my own code
• I have run the code locally and it works as expected
• I have commented my code, particularly in hard-to-understand areas

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