Adds a dependency to the dependency layer
        @type my_dep: L{Cdependency}
        @param my_dep: dependency object

def add_dependency(self,my_dep):
        """
        Adds a dependency to the dependency layer
        @type my_dep: L{Cdependency}
        @param my_dep: dependency object
        """
        if self.dependency_layer is None:
            self.dependency_layer = Cdependencies()
            self.root.append(self.dependency_layer.get_node())
        self.dependency_layer.add_dependency(my_dep)