Beautify function names

[qianyizh]

Things like:

• SelectionPolygonVolume::CropGeometry
• SelectDownSample, VoxelDownSample, UniformDownSample, ...
• ComputePointCloudToPointCloudDistance, ...

Move the output to the return part of functions. For all Geometry type, return std::share_ptr because Visualizer needs to share the ownership. But for std::vector, use the move and trivial copy mechanics to directly return.