Add custom functions for power and counting calls

[dilaraagac]

Added custom_power lambda function and custom_equation function with parameter validation. Implemented fn_w_counter to count function calls.

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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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