Sample consensus prerejective fixes

This PR fixes a couple of deficiencies in the sample consensus prerejective pose estimation class:

 - Instead of computing correspondences in each RANSAC iteration (as in SAC-IA), precompute them. This makes good sense when we want to turn up the number of iterations.
 - Better n-point sampler for generating relative poses (thanks to Troels Bo Jørgensen)
 - Removed a horrible condition making the algorithm continue without any evaluation in way too many cases
 - Avoided a duplicate squared Euclidean distance computation inside getFitness ()