Fixed open mode from w9 to w

[itan1]

When changing from GZip to CodecZlib the gzopen() method was replaced with open(), but the mode is still "w9". The gzopen() accepts a compression level number in the specified mode, but open() does not. Therefor I changed it to "w".

Fix for Issue #47

[Tokazama]

Thanks! This must have slipped in when switching to the new gz compressor. Would you mind just adding a quick test to make sure we don't accidently break writing compressed files in the future?

[itan1]

Yes, sure! Shall I also remove the files test/example4d.* while I'm on it? I think they are just duplicates of the test/data/example4d.* files which are used in runtests.jl?

[Tokazama]

Shall I also remove the files test/example4d.* while I'm on it? I think they are just duplicates of the test/data/example4d.* files which are used in runtests.jl?

I hope you don't mind, but I went ahead and changed the commit to solve this with a temporary directory that should get cleaned up once julia exits.

This looks good to me. Let me know if you want to make any other changes, otherwise I'll merge.

[itan1]

Ah, yes good idea. Thanks.

Ready to merge from my side 👍