normalize Py math code

Tests/Py/Fit/standalone_fits.py

@@ -13,14 +13,9 @@ class Rosenbrock:
         self.m_expected_params = [1, 1.0]
 
     def objective_function(self, params):
-        """
-        Returns the value of Rosenbrock function
-        """
         x = params["x"].value
         y = params["y"].value
-        tmp1 = y - x * x
-        tmp2 = 1 - x
-        return 100 * tmp1 * tmp1 + tmp2 * tmp2
+        return (1 - x)**2 + 100 * (y - x**2)**2
 
 
 def decaying_sin(params, x):