Simplified pcl::Registration::getFitnessScore()

getFitnessScore() is implemented quite complicated. To transform the input,
transformPointCloud() can be used and the point-to-point distance is allready
given by nn_dists[0] returned from nearestKSearch().
Merge pull request #338 from hdiethelm/fitness_score_fix

Simplified pcl::Registration::getFitnessScore()