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.

L3/Calculs Numériques/Interpolation.ipynb

@@ -534,7 +534,8 @@
     "def interp_vdm_poly(x, y):\n",
     "    \"\"\"Compute the coefficients of the interpolation polynomial.\"\"\"\n",
     "    if x.shape != y.shape:\n",
-    "        raise ValueError(\"x and y must have same dimension!\")\n",
+    "        msg = \"x and y must have same dimension!\"\n",
+    "        raise ValueError(msg)\n",
     "    return np.linalg.solve(interp_vdm_build(x), y)"
    ]
   },