Auto-detect source and reference for pc_align

[oleg-alexandrov]

pc_align expects the reference to be denser than the source point cloud, by the very nature of ICP. The user may not always know this, or may not care. Need a way of estimating automatically which of the two point clouds is denser, and use that as a reference. David suggested looking into lasinfo --compute_density (from lastools, not liblas).