Clears the data in the stream

def clear(self):
        "Clears the data in the stream"
        if isinstance(self.data, np.ndarray):
            data = self.data[:, :0]
        elif util.pd and isinstance(self.data, util.pd.DataFrame):
            data = self.data.iloc[:0]
        elif isinstance(self.data, dict):
            data = {k: v[:0] for k, v in self.data.items()}
        with util.disable_constant(self):
            self.data = data
        self.send(data)