Added symmtrical covariance functions

GPy/kern/constructors.py

@@ -20,6 +20,7 @@ from periodic_Matern32 import periodic_Matern32 as periodic_Matern32part
 from periodic_Matern52 import periodic_Matern52 as periodic_Matern52part
 from product import product as productpart
 from product_orthogonal import product_orthogonal as product_orthogonalpart
+from symmetric import symmetric as symmetric_part
 #TODO these s=constructors are not as clean as we'd like. Tidy the code up
 #using meta-classes to make the objects construct properly wthout them.
 
@@ -264,3 +265,12 @@ def product_orthogonal(k1,k2):
     """
     part = product_orthogonalpart(k1,k2)
     return kern(k1.D+k2.D, [part])
+
+def symmetric(k):
+    """
+    Construct a symmetrical kernel from an existing kernel
+    """
+    k_ = k.copy()
+    k_.parts = [symmetric_part(p) for p in k.parts]
+    return k_
+