Create calculator module with basic operations

## Description
Create a simple Python calculator module that provides functions for basic math operations.

## Requirements
- Implement add, subtract, multiply, and divide functions
- Each function should take 2 numbers as input and return the result

## Acceptance Criteria
- [ ] returns a + b
- [ ] returns a - b
- [ ] returns a * b
- [ ] returns a / b (handle division by zero)