Complete the calculator functionality - Issue #17

[add-hello-world]

Description:
Several functions in calculator.py are missing.
Please implement:

Basic operations (add, subtract, multiply, divide)

Memory functions (store_in_memory, etc.)

Variable assignment and expression evaluation

Factorial, sqrt, power

Make sure to handle divide-by-zero and invalid inputs gracefully.
Include simple unit tests in a new file test_calculator.py.