Implement custom mathematical functions and counter

[GunduzogluTurhan]

Added custom_power lambda and custom_equation function to perform calculations. Introduced fn_w_counter function to count calls.

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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