Progress bar for loading datasets

[eharkins]

Create a component updating on progress events from an event listener attached to the XMLHttpRequest, so that we can show the percent loaded of large datasets. (Maybe use vega? :))

[metasoarous]

I don't know if this is too important now that our dataset payloads are a lot smaller, so I'm going to put on ice. However, in case it becomes useful later, here's an example of how we might do this: https://zinoui.com/blog/ajax-request-progress-bar