Parsing utilities for saved metric map data

Tools to make accessing recorded metric map data easier. You can:
- load only the complete cloud from one saved sweep
- load all the complete clouds from sweeps saved in some folder (will automatically parse subfolders looking for sweeps)
- load all the complete clouds from sweeps recorded at some waypoint (takes as input the folder where the sweeps are located).
- similar functions for intermediate point clouds
- similar functions for intermediate position images
- also bugfixes and improvements in the simple xml parser and simple summary parser. 

**This PR does not affect the nodes running on the robot (i.e. the online system), they relate only to routines for loading saved metric map data.**