Replace BigTiff sample link (rebased onto dev_5_0)



This is the same as gh-1377 but rebased onto dev_5_0.

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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 hard drive 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.