Added intermediate registered transforms

This feature allows storing registered transforms between intermediate point clouds in a sweep (once the intermediate point clouds have been registered wrt each other). Useful as it allows storing both the original transformation (pose) of individual scans as given by the PTU and also the transform resulting from some registration algorithm. 

This PR builds on top of https://github.com/strands-project/strands_3d_mapping/pull/60 and https://github.com/strands-project/strands_3d_mapping/pull/61.