data_cube example todo

The following was previously a comment at the top of the file, but was removed to avoid noise when showing demo in etsdemo. I'm opening this issue to avoid using the info / to hopefully have the comments addressed someday.

```
# Outstanding TODOs:
# - need to add line inspectors to side and bottom plots, and synchronize
# with center plot
# - need to set the various image plots to use the same colormap instance,
# and that colormap's range needs to be set to min/max of the entire cube
# - refactor create_window() so there is less code duplication
# - try to eliminate the use of model.xs, ys, zs in favor of bounds tuples
```