Made rejectors work with ICP

Making correspondence rejectors actually work with ICP. As discussed in issue #324, this required adding PCLPointCloud2 setters to CorrespondenceRejector, with flags for whether or not they required source/target normals/points. As this requires a copy, it's a bit wasteful -- but short of a much larger refactoring job, it seems like the best choice to me. At least now the desired behavior is as expected -- let me know if I missed any Rejectors which use input/output clouds.

Note 1: CorrespondenceRejectorFeature does not fit this paradigm, so the user still needs to set this externally. The good news is any transform-invariant feature should be fine with this.
Note 2: Propagating this to JointICP as we speak
Note 3: the same trick can be done with CorrespondenceEstimation, so normals are always propagated (to those which require normals).