Move detrending from AverageModel to its own Filter

[oesteban]

In the context of #62, I'm simplifying the AverageModel (renamed as ExpectationModel to better capture that the statistic can be changed) and removing this code:

        # Regress out global signal differences
        if kwargs.pop("equalize", False):
            data = data.copy().astype("float32")
            reshaped_data = (
                data.reshape((-1, data.shape[-1])) if self._mask is None else data[self._mask]
            )
            p5 = np.percentile(reshaped_data, 5.0, axis=0)
            p95 = np.percentile(reshaped_data, 95.0, axis=0) - p5
            data = (data - p5) * p95.mean() / p95 + p5.mean()

This code was there to "detrend" global signals. It should be a filter and not part of the model.