Replace BigTiff sample link

The tiffcentral.com link has been broken since 15th Oct so I found a new link - this is the only sample set I could find that didn't say 'send us a harddrive to give you the data' and we do link to the same page in the 'see also' section so I'm going to assume it is safe to point people at downloading a zip from them. If not, I can just delete the link instead.