Append given layers to this onion

		:param layers: layer to add
		:return: None

def add_layers(self, *layers):
		""" Append given layers to this onion

		:param layers: layer to add
		:return: None
		"""
		for layer in layers:
			if layer.name() in self.__layers.keys():
				raise ValueError('Layer "%s" already exists' % layer.name())
			self.__layers[layer.name()] = layer