[registration] Parallelize SampleConsensusInitialAlignment with OpenMP

[mvieth]

The task is to parallelize SampleConsensusInitialAlignment using OpenMP, more specifically this for-loop: https://github.com/PointCloudLibrary/pcl/blob/master/registration/include/pcl/registration/impl/ia_ransac.hpp#L177 Should be pretty straightforward, with an OpenMP reduction on the variable error. For a parallelization example, see normal_3d_omp.h(pp). If there are questions, feel free to ask.

[zeel2104]

Is it open for contribution?

[mvieth]

@zeel2104 Hello, yes it is.