Apply an arbitrary function to ANTsImage.

        Args
        ----
        fn : python function or lambda
            function to apply to ENTIRE image at once

        Returns
        -------
        ANTsImage
            image with function applied to it

def apply(self, fn):
        """
        Apply an arbitrary function to ANTsImage.

        Args
        ----
        fn : python function or lambda
            function to apply to ENTIRE image at once

        Returns
        -------
        ANTsImage
            image with function applied to it
        """
        this_array = self.numpy()
        new_array = fn(this_array)
        return self.new_image_like(new_array)