Add test for custom operator

test/test.py

@@ -3,5 +3,16 @@ from pysr import pysr
 X = np.random.randn(100, 5)
 y = X[:, 0]
 
-equations = pysr(X, y, niterations=2)
+equations = pysr(X, y,
+                 niterations=100)
 print(equations)
+# Accuracy assertion
+assert equations.iloc[-1]['MSE'] < 1e-10
+
+y = X[:, 0]**2
+equations = pysr(X, y,
+                 unary_operators=["square(x) = x^2"], binary_operators=["plus"],
+                 niterations=100)
+print(equations)
+# Accuracy assertion
+assert equations.iloc[-1]['MSE'] < 1e-10