remove reliance on six

The amount of six which we were using was rather small. Move urlunsplit
into _compat. It turns out that iteritems was in fact *never* used :(.
download: hard-code zip and TIFF file paths

Having the expected zip and TIFF file paths customisable from the
command-line may have been a case of premature-configurability.
download: hard-code DEM path on server

Since the DEM zip filenames are hard-coded, it now makes little sense to
configure the path.