Add calculator core: constants, operations and tests

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 #19
Closes #20
Closes #21

Task: MIL-002#1
Task: MIL-002#2
Task: MIL-002#3

src/calc/operations.py

@@ -0,0 +1,63 @@
+"""!
+@file operations.py
+@brief Arithmetic operations and the dictionary that maps symbols to them.
+"""
+
+from collections.abc import Callable
+
+from calc.constants import (
+    SYMBOL_ADD,
+    SYMBOL_DIVIDE,
+    SYMBOL_MULTIPLY,
+    SYMBOL_SUBTRACT,
+)
+
+
+def add(n1: float, n2: float) -> float:
+    """!
+    @brief Add two numbers.
+    @param n1 The first number.
+    @param n2 The second number.
+    @return The sum of @p n1 and @p n2.
+    """
+    return n1 + n2
+
+
+def subtract(n1: float, n2: float) -> float:
+    """!
+    @brief Subtract the second number from the first.
+    @param n1 The first number.
+    @param n2 The second number.
+    @return The difference @p n1 minus @p n2.
+    """
+    return n1 - n2
+
+
+def multiply(n1: float, n2: float) -> float:
+    """!
+    @brief Multiply two numbers.
+    @param n1 The first number.
+    @param n2 The second number.
+    @return The product of @p n1 and @p n2.
+    """
+    return n1 * n2
+
+
+def divide(n1: float, n2: float) -> float:
+    """!
+    @brief Divide the first number by the second.
+    @param n1 The dividend.
+    @param n2 The divisor.
+    @return The quotient @p n1 divided by @p n2.
+    @throws ZeroDivisionError When @p n2 is zero.
+    """
+    return n1 / n2
+
+
+## Maps an operation symbol to its function. The functions are stored, not called.
+OPERATIONS: dict[str, Callable[[float, float], float]] = {
+    SYMBOL_ADD: add,
+    SYMBOL_SUBTRACT: subtract,
+    SYMBOL_MULTIPLY: multiply,
+    SYMBOL_DIVIDE: divide,
+}