add ghosts exchange in custom operator

hysop/backend/host/python/operator/custom.py

@@ -75,3 +75,5 @@ class PythonCustomOperator(HostOperator):
     def apply(self, **kwds):
         super(PythonCustomOperator, self).apply(**kwds)
         self.doutvar = self.func(*(self.dinvar + self.dinparam + self.doutparam))
+        for gh_exch in self.ghost_exchanger:
+            gh_exch.exchange_ghosts()