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.
[`initCompute()`](https://github.com/PointCloudLibrary/pcl/blob/master/common/include/pcl/impl/pcl_base.hpp#L139) of the `PCLBase` class (which is a base for all PCL classes) does this for you, so you only need to call it in the beginning.