Performance issues

[foobacca]

Loading a new file takes a while. It would be good to make it faster, and also to give some indication that data is loading (so it doesn't just look like the page has frozen).

A likely way to improve performance is to remove the SVG from the DOM at the start of loading, do all the work to add the paths, gradients etc to the map, and then put it back in the DOM. This saves the browser starting to render and re-render after every change. It would also mean we then have space to put a loading GIF up instead, until the page has reloaded.