Access the flow-field grid after applying queued affine transforms.

def flow(self)->FlowField:
        "Access the flow-field grid after applying queued affine transforms."
        if self._flow is None:
            self._flow = _affine_grid(self.shape)
        if self._affine_mat is not None:
            self._flow = _affine_mult(self._flow,self._affine_mat)
            self._affine_mat = None
        return self._flow