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/Statistical Learning/TP0_Intro_Jupyter_Python.ipynb

@@ -902,7 +902,7 @@
     }
    ],
    "source": [
-    "def divisible_by_3_and13(n):\n",
+    "def divisible_by_3_and13(n) -> None:\n",
     "    if n % 3 == 0 and n % 13 == 0:\n",
     "        print(n, \"is divisible by 3 and 13\")\n",
     "    else:\n",