fixed gicp guess handling

Denote final transformation result Y=guess*X. Gicp should either 1) optimize X from identity by comparing guess*source and target or 2) optimize Y starting from X by comparing source and target. Current GICP code messes these two approaches. In `computeTransformation`, it uses `base_transformation_ = guess` as the start of the optimization but use `output = guess * source` as the source cloud.

This PR fixed it to approach 1).