Add functions for custom power and equation with counter

[aysegulyildizz]

This file contains a custom power function, a custom equation function with parameter validation, and a function that counts calls to it.

Describe your changes

Checklist

• 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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