MIL-002 Calculator core.
- Add constants.py with operation symbols, answers, prompts, messages
and the ASCII title and calculator
- Add add, subtract, multiply and divide, and the OPERATIONS dictionary
that maps each symbol to its function without calling it
- Add tests for each operation, division by zero and the dictionary
Closes#19Closes#20Closes#21
Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3
