Pozo

- fix typo bug in combine_graphs()

- combine_tracks() now allows you to use new tracks and functions like add_tracks
(only the first argument can be a Data or Axis as well)

- Allow depth_unit to be None w/o Error
- add_tracks/axes/data now return the items finally added

- Add README.md
- Change `depth` to `show_depth` and `depth_range` to `depth` in interface