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/TP4_Ridge_Lasso_and_CV.ipynb

@@ -1084,7 +1084,7 @@
    ],
    "source": [
     "# We remove the players for whom Salary is missing\n",
-    "hitters.dropna(subset=[\"Salary\"], inplace=True)\n",
+    "hitters = hitters.dropna(subset=[\"Salary\"])\n",
     "\n",
     "X = hitters.select_dtypes(include=int)\n",
     "Y = hitters[\"Salary\"]\n",