Feature Request: UI enhancements

[johnkit]

Some suggested dataset-builder features -- some of them might already exist and I am not aware.

• Remove "Oceans Basins" from Xarray examples (pan3d/utils.py). That dataset applies labels to each voxel, and should be displayed with a "category" color map. Display in current pan3d looks like nonsense.
• Remove mybinder link from docs page. Sadly, the mybinder site is no longer unreliable. Ideally, we should have 2i2c set up a public binder instance.
• Provide some way to get the URL for a dataset loaded by pan3d config file. (It's a little surprising this is not in the config file itself.)
• Option to "lock" the camera so that you don't inadvertently change the view by mouse actions near the cell output.
• API to control the ruler component number of grids in each dimension
• API to set the font size and number of digits in ruler component labels
• Add descriptive text to example notebooks
• Check imports inside example notebooks