Add SVG pan library

[jmccrae]

I was looking at the https://lod-cloud.net/ diagram for something else and it just struck me it might make it easier for people to explore. Here is an example: https://media.ng-london.org.uk/svg/lod/ - Easy scroll wheel zoom and click and drag to explore.

[jpadfield]

The example is using code from: https://github.com/bumbu/svg-pan-zoom - but it is an earlier version (3.5.0) with a small edit to fix a error that popped up in the console, which is discussed in: bumbu/svg-pan-zoom#360