Estimate the min/max values of *data* by subsampling.

def _quickLevels(self, data):
        """
        Estimate the min/max values of *data* by subsampling.
        """
        while data.size > 1e6:
            ax = np.argmax(data.shape)
            sl = [slice(None)] * data.ndim
            sl[ax] = slice(None, None, 2)
            data = data[sl]
        return self._levelsFromMedianAndStd(data)