'add_zca_whitening.
Apply ZCA Whitening to data.
Arguments:
pc: `array` (optional). Use the provided pre-computed principal
component instead of computing it.
Returns:
Nothing.'

def add_zca_whitening(self, pc=None):

    self.global_pc.is_required = True
    self.global_pc.value = pc
    self.methods.append(self._zca_whitening)
    self.args.append(None)
