Use always euclidean distance

As supposed in: http://www.pcl-developers.org/In-Registration-Distance-set-with-setMaxCorrespondenceDistance-td5708244.html

Changing getFitnessScore() could be a bit dangerous, but at the moment, the documentation says:
[in]	max_range	maximum allowable distance between a point and its correspondence in the target (default: double::max)

So I think if somebody did not read the source code, he will expect that the distance is given euclidean and not square.