Implement custom functions and call counter

[helinHarman]

Added custom_power and custom_equation functions with type checks. Implemented fn_w_counter to track function 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

Screenshots (if appropriate)