Refactor error messages and function signatures across multiple notebooks for clarity and consistency

- Updated error messages in Gauss method and numerical methods to use variables for better readability.
- Added return type hints to function signatures in various notebooks to improve code documentation.
- Corrected minor grammatical issues in docstrings for better clarity.
- Adjusted print statements and list concatenations for improved output formatting.
- Enhanced plotting functions to ensure consistent figure handling.

M1/Numerical Optimisation/ComputerSession3.ipynb

@@ -248,7 +248,7 @@
     "    return result.x\n",
     "\n",
     "\n",
-    "def plot_perimeter(n):\n",
+    "def plot_perimeter(n) -> None:\n",
     "    optimal_angles = optimize_polygon(n + 1)\n",
     "    plt.figure(figsize=(7, 7))\n",
     "    t = np.linspace(0, 2 * np.pi, 100)\n",