register_translation (DFT) must return the cross correlation

[sciunto]

Description

In 0.12, register_translation does not return the cross correlation image. In the gallery, the cross correlation has to be calculated "by hand" http://scikit-image.org/docs/dev/auto_examples/transform/plot_register_translation.html#sphx-glr-auto-examples-transform-plot-register-translation-py
This means, that some parameters of _upsampled_dft have been pre-calculated.

In addition, another function match_template that can be used for similar purposes return the cross-correlation.

My suggestion would be to homogenize a little the API. For that, I suggest to return the cross correlation from register_translation. To me, this is critical as the user may want to check directly the quality of the correlation (and do not rely solely on the error estimate).

I already made the necessary modifications for a personal project. Therefore, I can backport if it sounds a good idea.