Add basic mathematical calculations utility module

[rwtamCaltech]

Summary

This issue tracks the addition of a basic mathematical calculations utility module (math_utils.py) to the repository.

Motivation

As the codebase grows with seismic processing scripts (PhaseNet, GaMMa), having a shared utility module for common mathematical operations will:

• Reduce code duplication across scripts
• Provide a consistent, tested set of math helpers
• Make future scripts easier to write and maintain

Proposed Implementation

Create a new file math_utils.py at the root of the repository that provides the following functions:

• add(a, b) — addition
• subtract(a, b) — subtraction
• multiply(a, b) — multiplication
• divide(a, b) — division (with zero-division guard)
• power(base, exp) — exponentiation
• square_root(n) — square root (with negative-input guard)
• mean(values) — arithmetic mean of a list
• median(values) — median of a list

Acceptance Criteria

• math_utils.py is created with all functions listed above
• Each function includes a docstring
• Division and square root guard against invalid inputs
• Functions are importable from other scripts in the repo