tiff2hdf.py: Use compression 'gzip' instead of 'lzf'

'lzf' is faster, though it takes twice as much space. Still, thanks
to occupying only about 1/10 of the uncompressed HDF5 file size, due
to the huge reduction of I/O, e.g. h5topng is still several times
faster on this file compared to uncompressed.