Add basic mathematical calculations utility module

[rwtamCaltech]

Summary

• Adds math_utils.py with 8 utility functions: add, subtract, multiply, divide, power, square_root, mean, median
• divide raises ValueError on zero divisor
• square_root raises ValueError for negative inputs
• All functions include docstrings

Closes #4

Test plan

• Import math_utils from another script and verify each function returns the expected result
• Confirm divide(1, 0) raises ValueError
• Confirm square_root(-1) raises ValueError
• Confirm mean([]) and median([]) raise ValueError