Optimization of expand in basic_modif.py

Making Expr immutable, and then optimize __hash__ with this

python_symb/IndependantTools/math_tools.py

@@ -0,0 +1,20 @@
+from __future__ import annotations
+from typing import *
+
+
+def gcd(a, b):
+    """
+    Greatest common divisor
+    work with any object that support modulo and comparison (contrary to math.gcd)
+
+    used in type : Fraction
+    """
+
+    if b > a:
+        return gcd(b, a)
+
+    if b == 0:
+        return a
+
+    return gcd(b, a % b)
+