4pcs registration method ...

...as proposed by Aiger et al. (2007, 4-Points Congruent Sets for Robust Surface Registration) and Theiler et al. (2014, Keypoint-based 4-Points Congruent Sets – Automated marker-less registration of laser scans).

Commit is together with a fast transformation estimation method to speed up the calculation using a minimum number of 3 points. This method simplifies the calculation of transformation parameters when dealing with a 3 point planar case.

I do realize, that a comparable contribution (fpcs by stheg) was done in June 2014, but since the additional method k-4pcs is inheriting a lot from its parent 4pcs, this commitment is only useful in combination.
Makes sense. But then let's immediately put it into an appropriate place. Where do you think it should go?