Added rotate_array.py

[davidw1281]

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
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• Documentation change?

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[davidw1281]

Could someone please review this?

[mindaugl]

Could reuse some functionality of Python to manipulate arrays.
For example, for the function rotate_array_n_space(), could return directly the combined slices of the array instead of using 'for' loops:

return nums[length - k:] + nums[:length - k]