Looks up the layer for this node based on the inputed layer name.
        
        :param      layerName   | <str>
        
        :return     <XNodeLayer>

def findLayer( self, layerName ):
        """
        Looks up the layer for this node based on the inputed layer name.
        
        :param      layerName   | <str>
        
        :return     <XNodeLayer>
        """
        for layer in self._layers:
            if ( layer.name() == layerName ):
                return layer
        return None