Add method to processors to toggle display in UI

[dale-wahl]

Adds a parameter to DataSet.get_available_processors() to only return processors that ought to be visible in the UI and a method to BasicProcessor that can be True/False.

This works in the cartographer to hide the base downloaders and instead only display the presets (which run the downloaders plus a basic cartographer to display the images in the UI). But it should allow us to run other processors via either the API or presets without necessarily displaying them to users (such as the processor I built to export network points from our Sigma.js previewer).