When user pass in argument for both decompression and multi-thread, print a warning message
to indicate that multi-threaded decompression is not supported.

move check to before nbWorker is assigned so we do not throw warning message when
-T0 is used.

add test case for multi-threaded decoding warning.
Expecation is for -d -T0 we will not throw any warning,
and see warning for any other -d -T(>1) inputs